This paper presents a numerical study of the solidification of pcm enclosed in a spherical shell. The mathematical model is based on pure conduction in the pcm subject to boundary conditions of constant temperature or convection heat transfer on the external surface of the spherical shell. The model is validated by comparison with available similar models and the agreement is found to be satisfactory. The model was then used to predict the effect of the size of the spherical shell, shell thickness, shell material, initial pcm temperature and the external wall temperature on the solidified mass fraction and the time for complete solidification.