This paper presents a numerical study on melting behavior of phase change material (PCM) in a horizontal double pipe heat exchanger. The shell side is filled with RT50 as PCM and water is used as heat transfer fluid (HTF) which flows through inner tube. The aim of the study is to investigate the effect of eccentricity as a geometrical parameter on melting behavior of PCM through downward movement of the inner tube. In addition, effective flow parameters such as mass flow rate and HTF inlet temperature are investigated on thermal storage performance. Enthalpy porosity method is used to modeling the phase change process. At the beginning of melting process, conduction is dominant heat transfer mechanism and by time pass natural convection will be the main heat transfer mechanism. Results show that by increasing eccentricity, the dominant area for the natural convection expands and phase front penetration velocity increases which leads to considerable decrease in melting time. By increasing inlet temperature from 70˚C to 75˚C and 80˚C, total melting time decreases up to 16% and 27% respectively. Although By Increasing Reynolds number from 1000 to 1500 and 2000, total melting time only decreases to 1% and 3%, respectively. These results show that Stefan number influences melting time more pronounced than Reynolds number.