Thermal energy storage plays an important role in a wide variety of industrial, commercial and residential applications. Phase change material (PCM) is used in these systems in order to store heat. Latent heat storage in a PCM is very attractive, because of its high-energy storage density and isothermal behavior during the phase change process. Increasing of building energy storage capacity can be achieved with decreasing the indoor air temperature variations so that the indoor temperature goes near comforting temperature for a long time that results in more comfort for human. In this paper, a mathematical model was developed based on an enthalpy formulation and the effect of PCM thickness on temperature distribution in PCM and melting fraction was investigated. Results show that melting time change linearly with the amount of PCM and lower thicknesses are suitable for a better PCM performance. (c) 2012 Elsevier B.V. All rights reserved.