Abstract: Rotating stall is a common unstable flow phenomenon in centrifugal pumps, usually occurring in part-load conditions. It will cause the performance instability of a pump, and even excite the resonance of the whole pump system in certain extreme cases. In this study, stall phenomena in a centrifugal pump under part-load conditions are investigated using CFD method, and the simulated results are compared with experimental measurements, including characteristic curves, internal flow structures, and frequency spectra. A new parameter of the impeller passage, namely blockage coefficient, is adopted to quantitatively evaluate the degree of rotating stall, and the large-scale vortex in a stalled channel is regarded as a stall cell. At the operating flow rate of Q = 0.35Q0 (where Q0 stands for the optional flow rate), five rotating stall cells are observed in the impeller, which propagate among different impeller channels at a rotating frequency lower than that of the impeller. From frequency spectrum analysis, the dominant frequency of these stall cells is about 23.4% of the impeller rotational frequency, or only 4.7% for each stall cell. At Q = 0.41Q0, a stationary stall phenomenon is observed in the impeller passages. In this case, the blockage coefficient of each impeller channel varies slightly during impeller rotation, and thus the stall of this type is relatively stable. Furthermore, the number of impeller blades plays a crucial role in determining the type of stall.