Almost all of the experiments on PCM melting rely on temperature measurement. However, based on the experimental evidence, the presence of a thermometer in the solid PCM affects the results. Because the solid phase covers the probe and in fact, the solid phase temperature is measured instead of the liquid phase. On the other hand, since the PCM sticks to the probe, the regime of melting can change from unconstrained to constrained melting. In this study, a new method is proposed to measure directly the liquid mass fraction of PCM during melting. This method can be categorized as a non-contact measurement method and is based on the image processing technique.In this method, images of the melting process are prepared under controlled conditions at successive times. By performing image processing techniques, the solid core boundary is determined and the enclosed area is calculated and divided by the solid phase initial surface area. The method was applied to PCM melting inside a cylinder. Meanwhile, the problem was also simulated numerically by the multi-phase method (VOF). The multi-phase method allows considering the change in the total volume of PCM (summation of volumes of both solid and liquid phases) due to the difference between solid and liquid densities. Good agreement was observed between numerical and experimental results. The RMSE was 1.8 %. Moreover, it was observed that the shapes and sizes of the solid core in experimental images and numerical results at different times match well. But for lower solid volume fractions, the solid core sinks deeper in experimental images in comparison with numerical results.