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 coupled to the output gear and actuated to close a path of a valve, and a selective power transmitter interrupting rotation of the output gear after the output gear is rotated at a predetermined angle by the rotational force of the input gear. After the valve is actuated to close the path, the driving force of the motor is not transmitted to the valve output shaft.