The stratified molten pool, consisting of an upper metal layer and a lower oxide layer, may form in the lower head of the reactor pressure vessels (RPV) during severe accidents of a pressurized water reactor (PWR) with in-vessel melt retention (IVR) strategy. It is therefore of paramount importance to quantify the thermal load distribution of the stratified structure on the RPV lower head. Motivated by such interest, the test facility SAMPO (StrAtified Molten Pool) is developed at CNPE to investigate the convection and heat transfer in stratified pools in a slice of semispherical vessel with the inner diameter in 1100 mm. In the present study two naturally stratified material pairs, i.e. water/thermal oil and nitrate salt (50 %NaNO3-50 %KNO3)/thermal oil, are used as the simulant materials of two types of stratified molten pool denoted by W-O and S-O, respectively. The upper layer has a free surface and the lower layer has an isothermal boundary due to the intensive cooling of water flowing through the external channel. The test results indicate that the profiles of central temperature and heat flux in the lower layers of molten pools are similar to those of a homogenous molten pool with a radiation upper boundary condition. The focusing effect is observed only in the S-O pool of nitrate salt/thermal oil, and the peak heat flux is strongly affected by heating power of the lower layer, thickness of the upper layer and the interlayer crust next the sidewall.