The F 1 motor is a rotating molecular motor that ensures a tight chemomechanical coupling between ATP hydrolysis/synthesis reactions and rotation steps. However, the mechanism underlying this tight coupling remains to be elucidated. In this study, we used electrorotation in single-molecule experiments using an F 1 bE190D mutant to demonstrate that the stall torque was significantly smaller than the wild-type F 1 , indicating a loose coupling of this mutant, despite showing similar stepping torque as the wild-type. Experiments on the ATPase activity after heat treatment and gel filtration of the a 3 b 3 -subcomplex revealed the unstable structure of the bE190D mutant. Our results suggest that the tight chemomechanical coupling of the F 1 motor relies on the structural stability of F 1 . We also discuss the difference between the stepping torque and the stall torque.