ATP synthase is a rotary motor which is composed of two portions: the ‘rotor’ F0, consisting of a c-ring, and the ‘stator’ F1, consisting of an α3β3 hexamer. In different species, the number of c-subunits which form the c-ring is varied from 10 to 14, whereas the α3β3 hexamer is fixed to be 3-fold symmetrical. We have numerically studied the rotational coupling between F0 with varied number of c-subunits and F1. It is found that, for any number of c-subunits,the rotor F0 advances 3 steps per revolution on average, which is determined by the period of F1, whereas the exact angular pausing positions are determined by the period of F0. When the symmetry of the c-ring of F0 is matched with the 3-fold symmetry of F1, the three steps have equivalent sizes. If not matched, the three steps become nonequivalent:both the step size and average dwell time are different for these steps.