A single molecule of F1-ATPase is by itself a rotary motor in which a central subunit, gamma, rotates against a surrounding stator cylinder made of alpha 3 beta 3 hexamer. Driven by the three beta subunits that hydrolyse ATP sequentially, the motor runs with discrete 120 degrees steps at low ATP concentrations. Over broad ranges of load and speed, the motor produces a constant torque of 40 pN.nm. The mechanical work the motor does in the 120 degrees step, or the work per ATP hydrolysed, is also constant and amounts to 80-90 pN.nm, which is close to the free energy of ATP hydrolysis. Thus this motor can work at near 100% efficiency.