The　nanocrystal　In2S3　(nc-In2S3)　has　been　used　as　a　visible　light　active　photocatalyst.　The　optical　absorption　indicated　a　narrow　band　gap　(E-g　=1.9　eV)　for　nc-In2S3.　Compared　with　TiO2-xNx,　the　decomposition　of　methyl　orange　using　nc-In2S3　revealed　enormously　enhanced　visible　light　activity.　The　center　dot　OH　during　the　photocatalytic　degradation　process　was　detected　by　tereplithalic　acid　photoluminescence　probing　technique　(TA-PL).　The　organic　intermediate　products　were　successfully　separated　by　liquid　chromatogram　and　subsequently　identified　by　an　electrospray　ionization　(ESI)　mass　spectral　technique.　The　possible　photocatalytic　mechanism　is　presented.