Melting phase relations of mantle peridotite and basaltic oceanic crust are essentially important for understanding the chemical evolution of the Earth's mantle. Here we discuss melting phase relations of KLB-1 peridotite and mid-ocean ridge basalt (MORB) at the uppermost lower mantle based on our recent experimental results. The results show the narrow solidus-liquidus interval within 200 K and that MORB has lower solidus temperature than KLB-1 peridotite at 27 GPa. We also estimated the density of partial silicate melts at high pressures and found that partial melts of KLB-1 peridotite may be gravitationally unstable at the uppermost lower mantle conditions, whereas partial melts of MORB may be stable. This suggests that the distribution of partial melts at the deep Earth`s mantle is different between KLB-1 peridotite and MORB.