Abstract: The NUMECA software Fine/Turbo is employed to investigate the three dimensional viscous flow in a diagonal fan impeller at the operating conditions with and without blade tip clearance respectively.The limiting streamlines,secondary flow patterns and velocity distributions of the two flow fields are analyzed by comparison between their numerical and experimental results.The transportation of low energy fluid in the flow passage,"jet/wake" structure and the formation of tip leakage vortex are revealed.The results show that the non-uniform flow wake induced by secondary motion and vortex always appear in the corner region of casing wall and pressure surface;and the effects of the tip clearance are to reduce the efficiency of the impeller and to enlarge the non-uniform level of the outflow field in the impeller.