A gas adaptor installed to an opening of a fuel tank comprises an inner cylinder; a top of the inner cylinder being extended with a flange; an center of the inner cylinder having a fuel receiving hole; an outer cylinder extending from an outer edge of the flange of the inner cylinder; an inner wall of the outer cylinder being assemble to an opening of a fuel tank; a tube extending from one end of the flange; the tube being communicable to an EEC; a guide hole extending from one end of the flange and communicable to the tube; a sub-gas chamber being formed between the inner cylinder and the outer cylinder; the sub-gas chamber being communicable to the guide hole. The sub-gas chamber is communicable to an evaporative emission control system (EEC) through the guide hole and the tube.