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Turbine blades deteriorate over operation and overall performance of turbine decreases due to dust entry, fuel impurities, chemical reactions with air impurities, sulphur in the fuel, incomplete combustion and unburned carbon production. In this article, the effects of blade surface roughness and geometry changes 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, Germany. Results showed a good agreement with the previous measurements. To find out the effect of blade deteriorations on turbine stage performance and flow field, blade deteriorations including blade surface roughness, tip clearance increment and fouling on vane and blade of turbine were applied in four different mass flow rates to the three-dimensional numerical model.