A motorcycle frame, which is adapted to be covered by a motorcycle casing with a footrest portion, said motorcycle frame comprising: a main pipe unit including a head tube and a head tube-supporting pipe, said head tube-supporting pipe having a top end portion that is connected to said head tube, and a bottom end portion that extends downwardly from said top end portion; a front frame unit including a pair of spaced apart front support pipes that are connected to said head tube-supporting pipe, each of said front support pipes having a downward section that extends downwardly from said head tube-supporting pipe, a bottom section adapted for extending in the footrest portion of the motorcycle casing, and a bent section connected between said downward section and said bottom section, said front frame unit further including an upper cross tube that is connected between said downward sections of said front support pipes and that is connected to said bottom end portion of said head tube-supporting pipe, and a lower cross tube that is connected between said bottom sections of said front support pipes, said front frame unit further including a receiving space that is bounded by said upper and lower cross tubes (42, 43), and said bent sections and said downward sections of said front support pipes; a rear frame unit including a pair of rear support pipes that are respectively connected to said bottom sections, each of said rear support pipes having an upward section that is connected to and extends upwardly from said bottom section of a respective one of said front support pipes, and an extension section that extends upwardly and rearward from said upward section; and a reinforcement unit including a pair of reinforcement members, each of said reinforcement members being disposed rearward of said upward section of a respective one of said rear support pipes and interconnecting said upward section of the respective one of said rear support pipes and said bottom section of the respective one of said front support pipes, said motorcycle frame being characterized in that said head tube-supporting pipe has a rectangular cross-section, said head tube-supporting pipe being composed of left and right pipe halves (323, 324) that are coupled together, each of said left and right pipe halves (323, 324) having a welding surface, said downward sections of said front support pipes being respectively welded to said left and right welding surfaces of said left and right pipe halves (323, 324). a main pipe unit including a head tube and a head tube-supporting pipe, said head tube-supporting pipe having a top end portion that is connected to said head tube, and a bottom end portion that extends downwardly from said top end portion; a front frame unit including a pair of spaced apart front support pipes that are connected to said head tube-supporting pipe, each of said front support pipes having a downward section that extends downwardly from said head tube-supporting pipe, a bottom section adapted for extending in the footrest portion of the motorcycle casing, and a bent section connected between said downward section and said bottom section, said front frame unit further including an upper cross tube that is connected between said downward sections of said front support pipes and that is connected to said bottom end portion of said head tube-supporting pipe, and a lower cross tube that is connected between said bottom sections of said front support pipes, said front frame unit further including a receiving space that is bounded by said upper and lower cross tubes (42, 43), and said bent sections and said downward sections of said front support pipes; a rear frame unit including a pair of rear support pipes that are respectively connected to said bottom sections, each of said rear support pipes having an upward section that is connected to and extends upwardly from said bottom section of a respective one of said front support pipes, and an extension section that extends upwardly and rearward from said upward section; and a reinforcement unit including a pair of reinforcement members, each of said reinforcement members being disposed rearward of said upward section of a respective one of said rear support pipes and interconnecting said upward section of the respective one of said rear support pipes and said bottom section of the respective one of said front support pipes, said motorcycle frame being characterized in that said head tube-supporting pipe has a rectangular cross-section, said head tube-supporting pipe being composed of left and right pipe halves (323, 324) that are coupled together, each of said left and right pipe halves (323, 324) having a welding surface, said downward sections of said front support pipes being respectively welded to said left and right welding surfaces of said left and right pipe halves (323, 324).