Mini centrifugal pumps having a diameter smaller than 100mm are employed in many fields. But the design method for the mini centrifugal pump is not established because the internal flow condition for these small-sized fluid machines is not clarified and conventional theory is not suitable for small-sized pumps. Therefore, mini centrifugal pumps with simple structure were investigated by this research. Splitter blades were adopted in this research to improve the performance and the internal flow condition of mini centrifugal pump which had large blade outlet angle. The original impeller without the splitter blades and the impeller with the splitter blades were prepared for experiment. The performance tests are conducted with these rotors in order to investigate the effect of the splitter blades on performance and internal flow condition of mini centrifugal pump. On the other hand, a three dimensional unsteady numerical flow analysis was conducted to investigate the change of the internal flow according to the rotor rotation. It is clarified from the experimental results that the performance of the mini centrifugal pump is improved by the splitter blades. The blade-to-blade low velocity region was suppressed in the case with the splitter blades. In addition to that, the unsteady flows near the volute casing tongue were suppressed due to the splitter blades. In the present paper, the performance of the mini centrifugal pump is shown and the unsteady flow condition is clarified with the results of the numerical flow analysis. Furthermore, the effects of the splitter blades on the performance and the unsteady internal flow condition are investigated. Mini centrifugal pumps having a diameter smaller than 100mm are employed in many fields. But the design method for the mini centrifugal pump is not established because the internal flow condition for these small-sized fluid machines is not clarified and conventional theory is not suitable for small -sized pumps. Therefore, mini centrifugal pumps with simple structure were investigated by this research. Splitter blades were adopted in this research to improve the performance and the internal flow condition of mini centrifugal pump which had large blade outlet angle. splitter blades and the impeller with the splitter blades were prepared for experiment. The performance tests are conducted with these rotors in order to investigate the effect of the splitter blades on performance and internal flow condition of mini centrifugal pump. On the other hand, a three dimensional unsteady numerical flow analysis was conducted to investigate the change of the interna It is clarified from the experimental results that the performance of the mini centrifugal pump is improved by the splitter blades. The blade-to-blade low velocity region was suppressed in the case with the splitter blades. In in the present paper, the performance of the mini centrifugal pump is shown with the unsteady flow condition is clarified with the results of the numerical flow analysis. Furthermore, the effects of the splitter blades on the performance and the unsteady internal flow condition are investigated.