Photocatalysis　is　an　emerging　and　promising　technology　for　indoor　air　purification.　This　purifying　method　features　the　good　performance　at　the　higher　pollutant　concentration,　in　contrast　to　the　lower　concentration.　This　feature　limits　the　technique　to　be　widely　used　in　indoor　air　purification,　in　that　the　contaminant　levels　of　indoor　air　is　usually　low.　In　this　study,　the　formaldehyde　(HCHO)　was　used　as　target　pollutant　and　the　nanometer　TiO2　particles　were　immobilized　on　the　surface　of　an　activated　carbon　(AC)　to　explore　the　influence　of　mass　transfer　due　to　the　absorption　of　AC　on　the　photocatalytic　reaction　rate　and　removal　rate　of　HCHO.　The　experimental　results　showed　that　the　photocatalytic　reaction　rate　was　improved　because　the　adsorbent　AC　can　absorb　the　pollutants　from　the　diluted　air　stream　to　the　surface　of　catalyst.　The　results　also　showed　that　the　photocatalytic　reaction　consisted　of　mass　transfer　control　process　and　photocatalytic　reaction　control　process　with　the　increasing　of　flow　velocity.　During　the　mass　transfer　control　process,　the　photocatalytic　reaction　rate　increased　with　the　increasing　of　flow　velocity,　while　in　the　photocatalytic　reaction　control　process　photocatalytic　reaction　rate　changed　very　little.　Compared　with　the　results　by　the　TiO2　/glass　film,　the　switch　point　of　the　photocatalytic　reaction　from　the　mass　transfer　control　process　to　the　photocatalytic　reaction　control　process　was　advanced　by　the　TiO2　/AC　film.　At　the　same　time,　the　indoor　HCHO　with　low　concentration　could　be　degraded　to　the　standard　range　which　is　specified　by　the　standard　of　indoor　air　quality　by　TiO2/AC　film.　Copyright　©　2009　by　ASME.