A crank structure for a bicycle pedal. A front outer thread of a cylindrical sleeve has a same rotary direction as an inner thread hole of the crank, and the rear outer thread has the opposite rotary direction to that of the front outer thread. The inner thread rotary direction of the cylindrical sleeve is the same as that of an outer thread of a bolt protruding from a pedal, and opposite edges of the two each has a hole, so that after the front outer thread of the cylindrical sleeve is secured to the thread of a threaded hole in the crank. A nut is used to lock the rear outer thread in order to limit the rotation. The bolt is locked into the inner thread of the hole of the cylindrical sleeve for the assembly in order to attain the effects of securely mounting and easily detaching the crank structure.