This study has been conducted to investigate the development of large-scale turbulent structures behind a backwardfacing step. The relative step height to boundary layer thickness of an incoming flow are H/δ0 = 0.63, and the momentum thickness Reynolds number is 3000. Mean and fluctuating velocity distributions were measured by the flying hot-wire technique with I-type multiple probe rake and the ordinary technique with X-type multiple probe rake. Proper orthogonal decomposition was employed to analyze the large scale structures in the separated shear layer. POD coefficients of the u- and v-component at the first mode were found to have a negative correlation with each other.