This paper presents a numerical study of heat transfer during melting of phase change material (PCM) encapsulated in spherical container of thermal energy storage, with dispersion of solid nanoparticles within phase change material. The finite volume method has been used to resolve the energy equation in both solid and liquid phases in transient regime. The mathematical model is based on the conjugate heat transfer in the PCM subject to constant temperature on the external surface of the spherical shell. The model developed in this study has been analyzed and compared with literature, and a good agreement is showed. A parametric study is conducted for different volume fraction of nanoparticles. The results show that the dispersion of nanoparticles increases the thermal conductivity of PCM and decreases the time of fusion.