We performed laboratory measurements of the heat flux and the interfacial temperature of two-layered thermal convection. Experiments show that the scaling law of the heat flux (Nu-Ra relation) can be classified by the buoyancy number B. The heat flux under B > 1 follows the well-known Nu-Ra relation. However, under B < 1, undulations develop at the interface, and heat flux is systematically enhanced. The interfacial temperature is determined only by the ratio of physical properties between the upper and lower layers. Our results suggest that the existence of less viscous and slightly denser D″ than the overlying mantle would enhance the heat transfer from the core to the mantle.