A motorcycle including: a front frame structure; a rear frame structure connected to a rear end of said front frame structure, and including a first frame unit that is connected to said front frame structure and that extends rearwardly, a second frame unit that is connected to said front frame structure, that is spaced apart from said first frame unit in a lateral direction, and that extends rearwardly; an engine mounted between said first frame unit and said second frame unit; and a rear shock absorber connected to said first frame unit of said rear frame structure; whereby: said first frame unit is formed as one piece and defines a receiving space that extends inwardly, said second frame unit is formed as one piece, and said rear shock absorber is mounted inside said receiving space; said first frame unit has a first upper frame connected to said front frame structure and extending rearwardly, and a first lower frame connected to and disposed below said first upper frame, and having a portion that is inwardly depressed and that cooperates with said first upper frame to define said receiving space; said second frame unit has a second upper frame connected to said front frame structure and extending rearwardly, and a second lower frame connected to and disposed under said second upper frame; each of said first upper frame, said first lower frame, said second upper frame, and said second lower frame is made of a non-iron material via a casting process; said first frame unit further has an absorber-connecting member formed on said first upper frame and connected to said rear shock absorber; said rear frame structure further includes a connecting unit connected between said first frame unit and said second frame unit; and said connecting unit has an upper cross frame having two ends that are respectively connected to said first upper frame of said first frame unit and said second upper frame of said second frame unit, a lower cross frame having two ends that are respectively connected to said first lower frame of said first frame unit and said second lower frame of said second frame unit, and a tail member having two ends respectively connected to a rear end of said first frame unit and a rear end of said second frame unit. a front frame structure; a rear frame structure connected to a rear end of said front frame structure, and including a first frame unit that is connected to said front frame structure and that extends rearwardly, a second frame unit that is connected to said front frame structure, that is spaced apart from said first frame unit in a lateral direction, and that extends rearwardly; an engine mounted between said first frame unit and said second frame unit; and a rear shock absorber connected to said first frame unit of said rear frame structure; whereby: said first frame unit is formed as one piece and defines a receiving space that extends inwardly, said second frame unit is formed as one piece, and said rear shock absorber is mounted inside said receiving space; said first frame unit has a first upper frame connected to said front frame structure and extending rearwardly, and a first lower frame connected to and disposed below said first upper frame, and having a portion that is inwardly depressed and that cooperates with said first upper frame to define said receiving space; said second frame unit has a second upper frame connected to said front frame structure and extending rearwardly, and a second lower frame connected to and disposed under said second upper frame; each of said first upper frame, said first lower frame, said second upper frame, and said second lower frame is made of a non-iron material via a casting process; said first frame unit further has an absorber-connecting member formed on said first upper frame and connected to said rear shock absorber; said rear frame structure further includes a connecting unit connected between said first frame unit and said second frame unit; and said connecting unit has said first frame unit is formed as one piece and defines a receiving space that extends inwardly, said second frame unit is formed as one piece, and said rear shock absorber is mounted inside said receiving space; said first frame unit has a first upper frame connected to said front frame structure and extending rearwardly, and a first lower frame connected to and disposed below said first upper frame, and having a portion that is inwardly depressed and that cooperates with said first upper frame to define said receiving space; said second frame unit has a second upper frame connected to said front frame structure and extending rearwardly, and a second lower frame connected to and disposed under said second upper frame; each of said first upper frame, said first lower frame, said second upper frame, and said second lower frame is made of a non-iron material via a casting process; said first frame unit further has an absorber-connecting member formed on said first upper frame and connected to said rear shock absorber; said rear frame structure further includes a connecting unit connected between said first frame unit and said second frame unit; and said connecting unit has an upper cross frame having two ends that are respectively connected to said first upper frame of said first frame unit and said second upper frame of said second frame unit, a lower cross frame having two ends that are respectively connected to said first lower frame of said first frame unit and said second lower frame of said second frame unit, and a tail member having two ends respectively connected to a rear end of said first frame unit and a rear end of said second frame unit.