During severe accidents in a nuclear power plant, the core melt (corium) may relocate to the lower head of the reactor pressure vessel (RPV). To prevent further progression of corium and massive release of radioactive materials out of RPV failure, the in-vessel melt retention (IVR) has been proposed in some advanced reactor designs, where a melt pool will be formed in the lower head and cooled by external flooding. Accurate estimation of the thermal loading over the lower head plays an essential role in the qualification of the IVR strategy. For this purpose, the present study investigates the temperature profile and heat transfer characteristics of a homogeneous melt pool and naturally stratified melt pools in the SAMPO test facility. Thermal oil and molten binary nitrates are employed as simulation materials of the upper and lower layer of the stratified melt pools, respectively. The results show that the maximum temperature and sidewall heat flux of the homogeneous melt pool may be overestimated. In addition, the stratified morphology of the melt pool also affects the crust formation: the sidewall crust range is larger in a stratified melt pool. A thinner upper layer yields a higher heat flux peak in the stratified melt pool.