The WO3 nanometer material can be prepared by sol-gel and improved sol-gel. We use it to make the direct-heating type gas sensor,and carry on the test to the low density NO2 gas. The result indicates that the WO3 material prepared by improved sol-gel has a very small particle size, its surface is loose,and it has very good sensitivities to low concentration even extremely low concentration NO2 gas. The element responds and resumes very quickly and has good selectivity. All elements have supreme sensitivity under lower working voltage, which means that the working temperature is relatively low. It helps to reduce the power consumption.