It　is　known　that　the　competitive　adsorption　of　water　vapor　onto　photocatalyst　causes　a　negative　effect　on　gas-solid　photocatalytic　reaction.　In　this　paper,　the　effect　of　water　vapor　on　the　microwave-assisted　photocatalytic　oxidation　of　C2H4　and　C2HCl3,　with　which　as　model　organic　pollutants,　was　examined　under　different　reaction　conditions.　The　results　showed　that　H2O　was　an　important　factor　to　the　photocatalytic　oxidation.　Because　polarization　of　different　substances　is　quite　different　in　microwave　field,　the　energy　of　microwave　was　mostly　absorbed　by　H2O　for　the　photocatalytic　reaction　with　water　vapor.　The　improvement　of　C2H4　photocatalytic　reactivity　was　attributed　to　desorption　of　water　vapor　from　the　catalyst　surface　in　microwave　field.　However,　the　result　of　C2HCl3　oxidation　was　opposite,　where　H2O　was　required　to　complete　the　reaction.