Surface roughness of steam turbine blades is increased during operation. This point has harmful effect on the performance of steam turbines. In this paper effects of surface roughness on performance of a steam turbine stage in two-phase flow conditions are investigated for different outlet pressures. To do so a numerical code has been developed to simulate two-phase non-equilibrium flow in 2D steam turbine geometry. An AUSM-van Leer hybrid scheme is used to calculate inviscid fluxes, the SST turbulence model for turbulence viscosity and Wilcox roughness model for implementation of roughness on the surface of turbine blade. To validate the present in-house code the experimental results of Bakhtar has been used. According to the results of the paper, effect of surface roughness variation on the performance loss in subsonic stages is more than that in supersonic outflows. For example, in subsonic outflow case (Pb= 24.25 kPa) when the roughness height increases from 5µm to 800 µm, the value of efficiency decreases by 15%. However, for supersonic outflow case (Pb= 14.55 kPa) the value of efficiency decreases by 10% for this roughness increase.