In this study performance of two vertical pumps has been improved by using CFD. The first pump is a vaporization sea-water pump which has 6 blade impeller and the specific speed of about 410. The second one is the cargo pump of a chemical product carrier. This pump is a submersible pump operating near the bottom of the cargo hold and has the specific speed of about 210. In order to get higher efficiency, the flow fields of the initial models of the pumps are simulated, and design directions are obtained by analyzing the flow fields. NPSH required values are also calculated to know whether the design models have good cavitation performance or not. Satisfactory results are obtained for the design models.