As　a　main　indoor　air　pollutant,　formaldehyde　was　chosen　as　the　aim　pollutant　Nano-structured　TiO2,　Ag　-TiO2　and　Ce-TiO2　thin　films　were　prepared　by　sol-gel　method　at　room　temperature　and　used　in　the　photo-catalyst　reactor　for　HCHO　decomposition.　The　effects　of　doped　Ag/Ce/TiO2,　relative　humidity,　oxygen　concentration,　initial　HCHO　concentration,　UV　light　wavelength,　TiO2　amount　on　HCHO　decomposition　were　investigated.　The　results　showed　that　doped　Ag　or　Ce　ions　could　enhance　the　photo-catalyst　ability.　35%　was　the　optimal　humidity　for　photo-catalyst　process　under　the　experimental　conditions.　Higher　concentration　of　oxygen　was　better　for　HCHO　removal.　Photo-catalytic　degradation　rates　decreased　with　increasing　HCHO　initial　concentration.　Illumination　using　a　254　nm　light　source　was　better　than　365　nm.　The　photo-catalytic　degradation　efficiency　increased　with　the　increasing　the　amount　of　TiO2　when　TiO2　amount　was　lower　than　70　mg.　The　reaction　rate　constant　(k)　of　TiO2,　Ag/TiO2,　Ce/TiO2　was　sequenced　k(Ce/TiO2)　＞　k(Ag/TiO2)　＞　k(TiO2),　meaning　that　Ce/TiO2　had　the　best　photo-catalytic　ability　among　the　catalysts.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.