Thermal stratification plays an important role in self-pressurization of liquid with external heating. To identify the thermal stratification phenomenon associated with external heating, an experiment was performed with a model fluid simulant, Novec 649. It was not necessary to heat the entire liquid because the vapor was generated outside the pressurization cylinder. Several heat loads and liquid levels were tested to study the effect of various operating conditions on the thermal stratification. A part of the liquid in the pressurization cylinder was drawn to the external heating system for vaporization, and the vaporized gas was recycled into the pressurization cylinder. The experimental results showed that the thermal stratification gradually increased along the axial direction down from the liquid surface. Analysis of the experimental results indicated that the contribution of thermal stratification from natural convection turned out to be negligible. It is concludes that the stratifying process can be explained by conduction, thermal expansion, and condensation of accumulated mass. Accumulated mass condenses on the surface of the liquid and is the most effective factor in the development of the thickness of the thermal stratification. The model for this study neglects the phenomena coupled with the three factors, but it may have minor effects on the stratification layer. To take into account the coupled phenomena, the correlations among those key factors must be specified precisely.