Recently, car pump market is positively growing in emerging countries as well as advanced countries. A increasing attention is recently drawn to electric vehicle but, still commercialization is expected to take a long time. This research is to study on the performance analysis for change in number of teeth of internal gear oil pump with three different numbers of teeth in the inner gear. After measuring real design features, modeling, velocity distribution and pressure distribution are conducted numerically. Pressure changes and volumetric efficiencies are compared with three different cases in order to analyze the performance for inner gears. Finally, optimum volumetric efficiency is determined with the sequence of case 1, case 2, and case 3.