Impellers with splitter blades have been used in turbomachinery design for both pumps and compressors. Increasing the number of blades increases the head of the pump, however it causes a decrease in efficiency due to the blockage effect of the blade thickness and firiction. The impellers with splitter blades between two long blades can be used to alleviate the serious clogging at the inlet of the impeller caused by more blades. In this study, impellers having a different number of blades (z=3, 4, 5, 6, and 7) with and without splitter blades (25, 35, 50, 60, and 80% of the main blade length) were tested in a deep well pump. The effects of the main blade number and lengths of splitter blades oil the pump performance have been investigated. While the number of main blades and the lengths of the splitter blades of a principal impeller were changed, the other parameters such as pump casing, blade inlet and outlet angles, blade thickness, impeller inlet and outlet diameters, were kept the same.