The WO3 based gas sensor is based on hall effect. The semiconductor material has selective adsorption to the gas that changes the electronic concentration which also raises hall coefficient , corresponding changes in the export of hall electromotive force. The experimental results show that the outputed voltage is from several millivolt to hundreds of millivolt and alters when the tensity of NO2 changed. The conclution is that this gas sensor based hall effect is a prospective gas sensor of new type.