A method of analysis using the equivalent thermal conductivity of the liquid phase is presented for the melting process of a phase change material (PCM) inside an isothermally heated horizontal cylinder. The equivalent thermal conductivity of the liquid phase during the melting process is estimated from the results for natural convection heat transfer between concentric horizontal circular cylinders. The melting process is, thus, analyzed as a pure heat conduction problem. Experiments are also performed using noctadecane as a PCM. The present analytical results for melting rates show good agreement with the experimental data for a wide range of Rayleigh numbers. It is also shown that the effect of natural convection on the melting process can be negligible for RaD < 105.