The　band　alignment　at　the　In2S3/Cu2ZnSnS4　heterojunction　interface　is　investigated　by　X-ray　photoemission　spectroscopy.　In2S3　is　thermally　evaporated　onto　the　contamination-free　polycrystalline　Cu2ZnSnS4　surface　prepared　by　magnetron　sputtering.　The　valence　band　offset　is　measured　to　be　0.46　+/-　A　0.1　eV,　which　matches　well　with　the　valance　band　offset　value　0.49　eV　calculated　using　＂transitivity＂　method.　The　conduction　band　offset　is　determined　to　be　0.82　+/-　A　0.1　eV,　indicating　a　＇type　I＇　band　alignment　at　the　heterojunction　interface.