Thermal energy storage is a practical and important way in conserving available energy and improving its utilization. Unlike the sensible heat storage method, the latent heat storage method provides much higher storage density, with a smaller temperature difference between charging and discharging heat phase. This work presents a numerical study of a latent heat storage exchanger filled with phase change material (PCM). Water flows by forced convection through the inner tube and transfers the heat to PCMs. A mathematical model based on the conservation energy equations was numerically resolved by a commercial FLUENT 6.3.2 code. Several numerical investigations were conducted in order to examine the impact of the key parameters: mass quantity of PCM used, number of pass and the mass flow rate of the water. This parametric study provides guidelines for system thermal performance and design optimization.