Pore morphology in partially molten peridotites strongly controls many physical properties of the Earth's deep materials. The dihedral angle of the olivine-silicate melt system is a key factor in understanding the physical behavior of the upper mantle. For a limited range of melt compositions, previous studies of wetting behavior have determined dihedral angles of 30-50 degrees. However, the dihedral angle of partially molten peridotites should be variable under a wide range of pressure and temperature conditions, because wetting behavior depends strongly on liquid structure and composition. The dihedral angles of olivine-silicate melt in partially molten peridotite were determined at pressures of 1 to 7 GPa and temperatures between 1473 and 1993 K. The dihedral angle decreased (approaching 0 degrees) with increasing temperature, which corresponds to completely wetted grain boundaries. This angle variation probably depends on a decrease in the solid-melt interfacial energy, which is a product of large change in melt structure. These results suggest that the presence of partial melt would have large influences on the physical properties of the upper mantle even if the melt fraction was very low. if grain boundary diffusion creep dominates in the asthenosphere, the shear strength of partially molten peridotites at asthenospheric pressure (similar to 3 GPa) would be much lower than that predicted from experiments at relatively lower pressure (similar to 0.3 GPa) because of the extremely low grain boundary contiguity. (C) 2009 Elsevier B.V. All rights reserved.