Thermal energy storage has recently attracted an increasing interest in thermal applications, such as the heating and cooling methods used in a house. Phase change material (PCM) is an important methods that has been proposed to achieve thermal energy storage. This paper numerically investigates the process of solidification and the melting of PCM in a vertical cylinder. The properties used in the simulations, include a melting temperature of approximately 27.4°C to 28.0℃, latent and sensible heat, thermal conductivity, and the density of solid and liquid states, all of which are based on a commercially available Octadecane. Transient three-dimensional simulations were performed using ANSYS CFX 12.0 software. The computational results showed how the phase-change process was conducted. These results were expressed in terms of the temperature and liquid mass fraction of the PCM.