A single molecule of F1–ATPase is by itself a rotary motor in which a central γ–subunit rotates against a surrounding cylinder made of α3β3–subunits. Driven by the three βs that sequentially hydrolyse ATP, the motor rotates in discrete 120° steps, as demonstrated in video images of the movement of an actin filament bound, as a marker, to the central γ–subunit. Over a broad range of load (hydrodynamic friction against the rotating actin filament) and speed, the F motor produces a constant torque of ca. 40 pN nm. The work done in a 120° step, or the work per ATP molecule, is thus ca. 80 pN nm. In cells, the free energy of ATP hydrolysis is ca. 90 pN nm per ATP molecule, suggesting that the F1 motor can work at near 100% efficiency. We confirmed in vitro that F1 indeed does ca. 80 pN nm of work under the condition where the free energy per ATP is 90 pN nm. The high efficiency may be related to the fully reversible nature of the F1 motor: the ATP synthase, of which F1 is a part, is considered to synthesize ATP from ADP and phosphate by reverse rotation of the F motor. Possible mechanisms of F1 rotation are discussed.