Spherical vesicles can change to various shapes such as oblate, prolate, stomatocyte-like, and starfish-like, according to the osmotic pressure difference between the inner and outer vesicles. The shape changes of vesicles are very important for understanding the activities of living cells. In this study, we investigated the process of vesicle shape change by carrying out dissipative particle dynamics simulations. We prepared spherical vesicles in which the difference between the numbers of lipids forming the inner and outer leaflets (Delta N) varied. If Delta N was small, with a decrease in the number of water beads inside a vesicle (N-w), a transformation from sphere to oblate, then oblate to stomatocyte-like was observed. If Delta N was large, a transformation from sphere to prolate, then prolate to tube-like occurred. The vesicles shape changes were in good agreement with the experiments. To investigate the mechanism of vesicle transformation in detail, we performed simulations by moving lipids between the inner and outer vesicle leaflets to vary Delta N. As a result, a transformation between the prolate and oblate vesicles was observed. These results indicated that the shapes of vesicles were determined by Delta N and N-w.