A tube for a bicycle, wherein the tube comprises: a body made of a material selected from one of a metallic material and a composition material and having an outer tube surface; an inner tube surface; an inner hole longitudinally formed through the body; and a tube thickness defined between the outer tube surface and the inner tube surface; an indented portion (20, 20A) radially recessed in the body from the outer tube surface toward the inner tube surface of the body and having a thickness being equal to the tube thickness of the body; an outer bottom surface (21, 21A); an outer annular surface (22, 22A) formed around the outer bottom surface (21, 21A) to form an opening gradually larger outward from the outer bottom surface (21, 21A) of the indented portion (20, 20A); a first round corner (23, 23A) being a curved surface smoothly connecting the outer tube surface of the body and the outer annular surface (22, 22A) of the indented portion (20, 20A); and an angle (α) formed between a tangent line, which is tangent to the outer annular surface (22, 22A), and a tangent plane, which is tangent to the outer tube surface of the body, being less than 90 degrees and greater than or equal to 15 degrees; and a machined hole formed through the outer bottom surface (21, 21A) of the indented portion (20, 20A) and communicating with the inner hole of the body and having an area being smaller than an area of the outer bottom surface (21, 21A) of the indented portion (20, 20A); characterized in that, the indented portion (20, 20A) has a recessed depth in the body, the recessed depth ranging from 1 time to 3 times of the tube thickness of the body; and a distance between two radially opposite theoretical intersections of outer edges of the outer annular surface (22, 22A) and the outer tube surfce is 1.3 times to 3 times of a distance between two radially opposite edges of the machined hole. a body made of a material selected from one of a metallic material and a composition material and having an outer tube surface; an inner tube surface; an inner hole longitudinally formed through the body; and a tube thickness defined between the outer tube surface and the inner tube surface; an outer tube surface; an inner tube surface; an inner hole longitudinally formed through the body; and a tube thickness defined between the outer tube surface and the inner tube surface; an indented portion (20, 20A) radially recessed in the body from the outer tube surface toward the inner tube surface of the body and having a thickness being equal to the tube thickness of the body; an outer bottom surface (21, 21A); an outer annular surface (22, 22A) formed around the outer bottom surface (21, 21A) to form an opening gradually larger outward from the outer bottom surface (21, 21A) of the indented portion (20, 20A); a first round corner (23, 23A) being a curved surface smoothly connecting the outer tube surface of the body and the outer annular surface (22, 22A) of the indented portion (20, 20A); and an angle (α) formed between a tangent line, which is tangent to the outer annular surface (22, 22A), and a tangent plane, which is tangent to the outer tube surface of the body, being less than 90 degrees and greater than or equal to 15 degrees; and a thickness being equal to the tube thickness of the body; an outer bottom surface (21, 21A); an outer annular surface (22, 22A) formed around the outer bottom surface (21, 21A) to form an opening gradually larger outward from the outer bottom surface (21, 21A) of the indented portion (20, 20A); a first round corner (23, 23A) being a curved surface smoothly connecting the outer tube surface of the body and the outer annular surface (22, 22A) of the indented portion (20, 20A); and an angle (α) formed between a tangent line, which is tangent to the outer annular surface (22, 22A), and a tangent plane, which is tangent to the outer tube surface of the body, being less than 90 degrees and greater than or equal to 15 degrees; and a machined hole formed through the outer bottom surface (21, 21A) of the indented portion (20, 20A) and communicating with the inner hole of the body and having an area being smaller than an area of the outer bottom surface (21, 21A) of the indented portion (20, 20A); characterized in that, the indented portion (20, 20A) has a recessed depth in the body, the recessed depth ranging from 1 time to 3 times of the tube thickness of the body; and a distance between two radially opposite theoretical intersections of outer edges of the outer annular surface (22, 22A) and the outer tube surfce is 1.3 times to 3 times of a distance between two radially opposite edges of the machined hole.