Abstract: A bicycle crank driven chain wheel assembly uses a special-shaped chain wheel. The chain wheel has portions of different driving radii. The portion having smaller driving radii is engaged in transmission when the bicycle runs on slopes, so that the bicycle rider can drive the bicycle with smaller force. The portion having larger driving radii carries out transmission when the bicycle runs on flat road, so that the bicycle can be speeded up. The crank is mounted in such a manner that it can rotate with respect to the chain wheel within predetermined range of angle, so as to select the corresponding portion of the chain wheel to be engaged in transmission in accordance with the resistance to motion of the bicycle or with the condition of the road on which the bicycle runs. Thus, a variable transmission can be automatically achieved corresponding to the resistance to motion of the bicycle or to the condition of the road.