The structure of metal oxide thin films can greatly affect their gas sensing properties. However, the structure-property relationship has not been fully established for the NO2 sensing properties of WO3 thin films. In this study, WO3 thin films with two distinct structures have been prepared by the sol-gel technique via employing two different precursor sols. Dense and porous WO3 thin films were successfully prepared as tungsten and WCl6 was used as the precursor source, respectively. Both WO3 thin films are exclusively sensitive to NO2 as low as 80 ppb. Porous WO3 thin films exhibit higher response toward NO2 but slower response/recovery kinetics as compared to the dense ones. Hall measurement and electrochromic coloration confirm the higher carrier mobility in the porous WO3 thin films. The response/recovery kinetics is found to be determined by the gas-solid reaction (receptor function), which is combinedly influenced by the porosity and grain size of the WO3 thin films.