The　effect　of　magnetic　field　on　the　photocatalytic　oxidation　of　benzene　and　ethylene　contained　in　air　over　Pt/TiO2　was　studied　under　365　nm　irradiation　in　the　range　of　magnetic　field　intensity　of　＜=　0.2　T.　It　was　found　that　when　the　UV　lamp　was　near　to　or　under　a　magnetic　field,　the　magnetic　field　could　enhance　the　photocatalytic　conversion　and　mineralization　of　benzene　and　ethylene.　However,　no　effect　of　magnetic　field　on　the　reactions　was　observed　when　the　UV　lamp　was　far　away　from　the　magnetic　field　and　the　light　was　transmitted　onto　the　reactor　by　a　focusing　fiber.　In　the　former　case,　the　observed　enhance　of　photocatalytic　oxidation　of　the　two　reactants　was　a　result　of　the　increase　of　brightness　of　the　UV　lamp　under　magnetic　field.　This　is　because　that　high　photo　intensity　provided　high　light　energy　for　the　reaction　and　led　to　increase　of　the　reaction　temperature.　In　the　latter　case,　intensity　of　UV　light　wasn＇t　disturbed　by　the　magnetic　field,　applying　magnetic　field　had　no　effect　on　the　two　photocatalytic　reactions.　The　results　showed　that　there　was　no　any　inherent　effect　on　the　gas-solid　phase　photocatalytic　reaction.