In order to understand the characteristics of an impeller independently, it is necessary that the impeller is tested without a diffuser. Using this method, the surge may be caused either by the inducer stall or impeller stall. Two types of impellers, with different inducer blade-to-blade shape, were employed to investigate the critical condition of stall and surge. Employing a one-dimensional-flow analysis within the impeller, various parameters for stall and surge were investigated. In this study, the relative velocity reduction between the inlet and throat of the inducer, and between the inlet and exit of the impeller were examined in comparison with the inducer throat Mach number.