The effect of aspect ratio on the melting process of PCM (polyethylene glycol 1500 (PEG 1500)) in a rectangular enclosure was investigated using CFD modelling. The CFD model was validated by experimental measurement performed for one case of aspect ratio of 2.31 with height = 155 mm, width = 67 mm and depth = 200 mm. The model was then used to study the melting percentage profiles with time for different aspect ratios. The computational domain was based upon a rectangular enclosure filled with PEG 1500 that was maintain at a constant temperature difference of 60 °C by heating one side to 80 °C and while the opposite side was maintained at 20 °C. The study was conducted in two phases: i) Constant height enclosure: the width of the enclosure was changed to give the desired change in aspect ratio while keeping depth constant; ii) Constant volume enclosure: the aspect ratio changed by varying the height and width to give the desired aspect ratio while keeping depth and volume of PCM constant The experimental case was used as a reference for both constant volume and constant heating wall area.