Unsteady effect of rotor-stator interaction on turbine stage performance was investigated for two cases of axial spacings between stator and rotor, i.e. large and small axial spacings. Unsteady RANS simulations were performed to clarify several interesting features of the unsteady three-dimensional flow field in the turbine stage. Simulation results showed that the stator wake was convected from pressure side to suction side in the rotor. As a result, secondary flow was periodically generated by the wake passing through the rotor passage, and then the passage vortices in the rotor near the endwall were suppressed and fluctuated in the radial direction downstream of the rotor. It was found that turbine stage efficiency with the small gap was higher than that with the large gap because the wake entered the rotor before mixing.