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 storage tests for LNG cold energy by freezing n-pentane as a PCM (phase-change material) in a latent heat storage unit with finned tubes, which was used in the liquefaction tests for LNG two-phase flow in our previous work. The behavior of the PCM freezing process outside the finned tube is explained from the investigation of the overall performance of the storage unit and the heat transfer characteristics around the finned tubes. It is shown that the ratio of stored cold energy to storable energy, F, is effective for correlating the experimental results in the freezing process as well as in the melting process. Furthermore, the thermal conductance of the finned tubes in the freezing process, which was smaller than that in the melting process, is represented by a simple cylindrical model.