A valve actuator includes a motor and a gear assembly including an input gear rotated by driving force of the motor, an output gear receiving rotational force of the input gear, and at least one power transmission gear transmitting the rotational force of the input gear to the output gear. The valve actuator further includes a valve output shaft actuated to close a path of a valve, and a selective power transmitter selectively transmitting the rotational force of the output gear to the valve output shaft. The selective power transmitter includes a cam portion provided in at least one of the output gear and the valve output shaft. Accordingly, after the valve is actuated to close the path, the driving force of the motor is not transmitted to the valve output shaft.