A speed change device of a bicycle comprises a body having a ring clamping portion and a driven portion; the driven portion being connected to a chain driving sheet; an L shape driven arm pivoted to the body; one end of the driven arm being installed with a connecting portion; and another end thereof being installed with a pivot portion; the pivot portion being pivoted to the chain driving sheet; wherein the chain driving sheet is driven by the driven arm to achieve the object of moving a chain received in the chain driving sheet and to change speed of a bicycle. A bending portion of the L shape driven arm has a cambered shape. The connecting portion of the driven arm receives a speed change rope.