A simple computational model for isothermal phase change problem was based on the enthalpy formulation. The temperature field and liquid fraction field were decoupled of the computational model to solve the temperature of the control volume separately which was melting/freezing or not. The computational model was verified with a test problem of a rectangular container. The agreement of the computation results with the experimental data indicates this method could be used to estimate the model-s isothermal phase change quickly and effectively. The numerical analysis of the melting time of phase change material for rectangular and cylindrical containers was then performed using the computational model. The results show that the phase change material of rectangular container requires nearly half of the melting time as for the cylindrical container of the same volume and characteristic size.