The influence of the tip clearance height of turbine rotor blades on the aerodynamic performance and secondary flow losses of an axial turbine stage were investigated by numerical simulation and different tip clearance heights were taken into account.Results show that the efficiency of the turbine stage would decrease markedly due to the increase of the tip clearance height.The efficiency would decrease 1.8% as the tip clearance height increased 1% of blade relative height.Analyses of the rotor flow field indicated that as the height if tip clearance increased

the distribution of flow losses changed in the upper passage of the rotor.The most losses were induced in the region between the middle of the rotor blade and the trailing edge

while secondary flow losses increased most behind the trailing edge.Mixing losses were the main factors which led to the efficiency decreased as the height of tip clearance increased.