Abstract: This invention relates to a pedal mechanism for a bicycle with variable leverage pedal cranks. Two guide pulley assemblies fixedly mounted on the frame of a bicycle each comprise two relatively rotatable members cooperating with an arrangement of a chain sprocket, a flange, pins and the pedal cranks so that the lengths of the cranks varies between minimum and maximum values during each rotation of the cranks. The chain sprocket has fifty-two teeth and a free wheel sprocket eighteen teeth, the driving ratio being 2.88. Owing to the variation in driving arm of the cranks, the power consumption can be lowered and speed can be increased up to 20% compared with conventional pedal mechanisms.