This study investigates the thermodynamic behavior that occurs when cryogenic liquids, such as liquefied natural gas (LNG) and liquid hydrogen, are stored. The experiment was conducted with liquid nitrogen and cryogenic liquid under 0.024 W/(m K) using vacuum insulation characterized by a thermal conductivity coefficient similar to that of the polyurethane foam commonly installed in LNG storage tanks. The experimental conditions are found to be similar to those of a commonly used type-B (membrane type) LNG storage tank based on the calculations of the thermal aspect ratio, which indicates the heat ingress ratio from the sides of the tank compared to the total heat ingress. Cryogenic liquid stored in a tank generally shows different thermodynamic behaviors because of the heat ingress from the outside. Compared with those predicted by the homogeneous model, the experimental results indicate different behaviors because of thermal stratification. The results particularly show that thermal stratification is highly correlated with the thermal aspect ratio. Moreover, the higher-level fraction results in faster pressure increases, which is in contrast with the result predicted by the homogeneous model. The boil-off gas rate generated is significantly less than that predicted by the general calculation in the homogeneous model even if much boil-off gas is generated in the early stage in a system with a high-level fraction. In conclusion, this study shows that the thermodynamic behaviors resulting from thermal stratification are significantly different from those predicted by the homogeneous model. Therefore, the related equipment must be designed and operated based on these considerations.