The band alignment of Zn_0.9 Co_0.1O/ZnO thin film heterostructure was investigated using photoelect ron spectroscopy. The ZnO, Zn_0.9 Co_0.1O and Zn_0.9Co_0.1O/ZnO thin film heterostructure has been prepared using RF ion beam sputtering, The band offset of prepared heterostructure were calculated form valance band maximum and core level peaks. The valance band and conduction band offset for Zn_0.9Co_0.1O/ZnO heterostructure thin film were found to be 0.36 eV and 0.51 eV, respectively, which indicates type-II b and alignment in the Zn_0.9Co_0.1O/ZnO heter O structure.