Application of an energy storage system to the LNG vaporization process can level the variation in cold energy generation rate arising from daily and seasonal fluctuations in natural gas consumption, thereby promoting LNG cold energy utilization. In order to commercialize a BOG (Boil-off Gas) re-liquefaction process with a cold energy storage system, we have conducted liquefaction tests for LNG two-phase flow by melting solidified n-pentane as a PCM (phase-change material). Using a latent heat storage unit with finned tubes of similar length to those in a commercial plant, we investigated the overall performance of the storage unit and the heat transfer characteristics around the finned tubes. As a result, LNG and PCM phase change processes were clarified inside and outside the tubes, respectively. Liquefaction time of BOG in the storage unit was approximately in inverse proportion to the duty of the unit, but it became shorter with the increase in the duty since it was more influenced by PCM solid remaining near the outlet of the storage unit. Furthermore, the thermal conductance of the finned tubes of the storage unit was estimated by using the correlations for heat transfer reported in previous studies.