Thermal performance and phase change stability of palmitic and stearic acids as a latent heat energy storage material has been study experimentally. In the experimental study, the thermal performance and heat transfer characteristics of the palmitic and stearic acid was tested and compared with other studies given in the literature. The present study included some parameters such as transition times, temperature range, and propagation of the solid-liquid interface as well as heat flow rate effect on the phase change stability of palmitic and stearic acid as phase change materials (PCMs). The results also show that better stability of the stearic and palmitic acids were accomplished at low inlet water temperature compared with the obtained results at high inlet water temperature. We also observed that while the heat exchanger tube is in the horizontal position, the PCMs have more effective and steady phase change characteristics them in the vertical position. Average heat storage efficiencies were found to be 50.3 and 53.3% for stearic and palmitic acids, respectively.