We have fabricated $WO_3$ thick film gas sensor under various firing conditions in order to study gas sensing properties in terms of the variation of microstructure and non-stoichiometric structure of gas sensing layer. $WO_3$ paste mixed homogeneously with organic vehicle was coated by screen printing method on alumina substrate composed of Au electrode and $RuO_2$heater on each side. To change filing condition, sensing materials were fared at 600-$800^{circ}C$ for 1 hour and refired at $700^{circ}C$ for 1 hour in the mixtures of $_Ar/O2$gas. In the result of heat-treatment, $WO_3$ gas sensor fared at $700^{circ}C$ showed best gas sensing properties of 210 gas sensitivity and 2 second response time and the best firing environment was 40-50% of $Ar/O_2$gas.