A valve actuator includes a motor and a gear assembly that receives driving force of the motor and controls a valve. In the gear assembly, a power transmission gear for transmitting rotational force of an input gear to an output gear includes a selective power transmission unit. When the output gear is in physical contact with a stopper, the power transmission gear transmits the rotational force of the input gear to the output gear is blocked. Accordingly, even after the valve is actuated to close a path, the rotational force of the input gear is not transmitted to the output gear even though the motor is continuously driven.