Recently, latent heat thermal energy storage system has been used in order to utilize the middle-temperature (100 - 200 °C ) factory-waste heat. In the range of middle temperature, erythritol is a suitable PCM, which has melting point at 119 °C , and latent heat of 340 kJ ⁄ kg . In this experiment, silicon oil as a heat transfer fluid injects from the nozzle plate of the bottom of a heat storage vessel. It flows to the upper direction through PCM region and heat is directly transfered between PCM and oil. In this study, the melting and solidification behavior between oil and PCM were visualized in order to investigate the characteristics of heat storage and release at difference flow rate of heat transfer fluid (1.0 - 4.0 kg ⁄ min ) and packed height of PCM (0.23, 0.45, 0.60 m ). The experimental result shows that the amount of heat storage and release is influenced by the flow rate of oil and the packed height of PCM. It is found that the volumetric overall heat transfer coefficient increases when the flow rate of the oil increases. During solidification process, the volumetric overall heat transfer coefficient decreases with increasing the packed height of PCM, while, it is not different in the melting process.