Abstract: Particle image velocimetry(PIV) system,pump experimental facility and test scheme were introduced.An experimental investigation of flow in the impeller was conducted at three different flow rates on three measurement planes by means of PIV.The velocity vector distribution was obtained.The velocity distribution in circumferential and axial directions and the influence of flow rate on the distribution were investigated.The results indicate that,the velocity near the suction surface decreases slightly,and that near the pressure surface changes dramatically with decreasing flow rate.The fluid flows back near the pressure surface when the flow rate reaches 0.6Q_d.At the outlet of impeller,the flow rate has no significant influence on the direction of relative velocity.The velocity in the impeller passage distributes homogeneously in axial direction.The absolute velocity shows an increasing trend in radial direction.The circumferential component of nondimensional absolute velocity decreases from the pressure surface to the suction surface,and also increases with the decline of flow rate.The results provide valuable information for the design of impeller and experimental study on flow in the centrifugal pump.