In this study the effect of longitudinal fins in a double pipe heat exchanger containing PCM is examined during charging process. Therefore eight rectangular fins are mounted around the HTF (heat transfer fluid) carrying tube. Two fins' heights and three Stefan numbers are chosen in order to examine the effect of these two parameters on thermal performance of the heat exchanger. In order to study the heat exchanger experimentally, three sections are chosen and several thermocouples are located in the sections. Observing thermal distribution, melting front, temperature and velocity contours and liquid fraction versus time, detailed behavior of melting process is explained. The process of heat absorption versus time is also discussed. Results show that fins extension leads to the less melting time and deeper penetration of heat. It is also shown that heat absorption power of the heat exchanger is a function of fins height at initial steps of the charging process. (C) 2015 Elsevier B.V. All rights reserved.