A pump is a machine that transports liquid or gas fluid through a pipe by pressure action, or pushes fluid in a low pressure container through a pipe into a high pressure vessel. Nowadays, pumps are used wherever fluid is used, ranging from farms, mining and civil engineering sites, factories and homes. Since the performance of the pump is influenced by the shape, the research has been conducted to improve the performance by changing the shape. The present study especially focused on the shape of the impeller and investigated the performance of the pump according to clearance between the flap and impeller blade. Numerical analysis is performed using Ansys CFX, a commercial code. The results are shown through performance curves. The velocity distributions and three-dimensional streamline are graphically depicted.