The shrouded and unshrouded impellers are two typical kinds of impellers, which are widely utilized in centrifugal compressors of various applications. Centrifugal compressors with unshrouded impellers are generally recognized to display inferior performance to the shrouded impellers with the same geometry. In this paper, a comparative experiment shows some results inconsistent with conventional cognition. Measured performance indicates that the peak efficiency of the centrifugal compressor with an unshrouded impeller is higher than the shrouded one, where the two impellers have the same geometry of meridional profile and blade central plane, and matched the same vaneless diffuser and volute. In order to explore the causes of this divergence, the effects of factors such as blade thickness, surface roughness of components, tip clearance and sealing leakage characteristics on performance are analyzed by CFD code. Numerical results show that reasonable reduction in the blade thickness and improvement on the surface quality of the impeller could effectively increase the peak efficiency and the choke mass flow rate of the shrouded impeller. The unshrouded impeller with arbitrary blade surfaces would be deformed under the action of centrifugal force to achieve a small tip clearance during operation, and then obtains higher efficiency at design speed. The research results are helpful to evaluate the performance potential and sensitive design parameters of shrouded and unshrouded impellers.