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The performance of turbine section of a gas turbine deteriorates over operation because of working in high temperature conditions and characteristics of the entry gas. On the other hand, due to complexity of the flow field within the turbine, three-dimensional analysis is required. This paper presents a numerical study of roughness effects on turbine flow field and performance. In this paper, effects of blade surface roughness caused by operation conditions on turbine performance were numerically calculated. Numerical calculations were carried out for the fourth stage of an axial turbine which was experimentally tested in the technical university of Hannover, using ANSYS software. Calculated results were verified with the measured data and showed a good agreement. To find out the effects of blade surface roughness on turbine stage performance and flow field, Two equivalent sand-grain roughness heights of 106㎛ (transitionally rough regime) and 400㎛ (fully rough regime) in four different mass flow rates were considered. Results showed that summation of efficiency reductions of the rough stator and rough rotor approximately equals to that of the totally rough stage for each roughness height and effect of stator roughness on efficiency reduction is same as the effect of rotor roughness on stage efficiency.