Embodiments of mechanisms for charging a gas into a cassette pod are provided. A method for charging a gas into a cassette pod includes loading at least one semiconductor wafer into a housing of the cassette pod after the at least one semiconductor wafer is processed by a processing apparatus. The method also includes removing the cassette pod from the processing apparatus by a transporting apparatus to a predetermined destination. The method further includes charging a gas into an enclosure in the housing of the cassette pod from a gas supply assembly disposed on the housing.