The purpose of this study is to investigate the influence of the aspect ratio and Grashof number on the convection diffusion phenomena associated with solid liquid phase transition processes during PCM solidification within a rectangular cavity. The applied two-dimensional physical model is based on the enthalpy method approach. The numerical solutions are obtained with finite volume method and validated with existing numerical and experimental studies of natural convection in phase change material. Numerical analysis are performed using water as phase change material, for an aspect ratio range 0.5 to 4. The left and right walls of the cavity are maintained at hot and cold temperatures, respectively, while the upper and lower surfaces are considered adiabatic. From this study, it is found that PCM initially liquid evolves to thermal stationary regime where solid and liquid regions are present. The solidification phase change process depends considerably on the thermal and geometrical parameters of the system. Additionally, it is demonstrated that the physical approach based on a purely conductive model remains valid to cases of low values of Grashof number and aspect ratio. The findings of this research may contribute to the elaboration of an efficient modeling of PCM heat storage systems.