In this investigation the coupled phase change and heat transfer in a spherical capsule partially filled with sodium nitrate is numerically investigated during the solidification process. The finite volume method is used to compute energy and linear momentum equations along with the enthalpy-porosity method to track the melting front. The volume of fluid model is used to track the transient behavior of the PCM-Air interface. A spherical capsule under the gravitational field with inner radius, R i , is initially filled by a determined volume of solid NaNO 3 , the remaining internal volume is completed with air. For time t>0, a constant temperature boundary condition T w , which is lower than the solidification temperature of the PCM, is imposed at the outer surface of the shell. Transient melt fraction and temperature distributions within the spherical capsule are analyzed for different parameters such as Grashof, Stefan and Prandtl numbers in order to present an elaborate discussion of the thermal performance of the system. The results showed that the total solidification time decreases up to 25% when the temperature difference between the PCM melting point and the outer wall changed from 5 to 15 °C.