Many physical properties of liquid-bearing rocks in the deep Earth's interior are strongly controlled by its wetting behavior. We report experimental results on the variation of dihedral angle and micro structures in the forsterite-H2O system at 1200 degrees C and pressures ranging from I to 13 GPa. The dihedral angle of the system decreases systematically with pressure and above the mantle transition zone (similar to 400 km depth) it becomes approximately 0 degrees, corresponding to completely wetted grain boundaries. This condition is probably reached because of a decrease in the solid-liquid interfacial energy with pressure due to the dramatically increased solubility of silicates in the liquid. These results suggest that the presence of partial melt would have drastic influences on physical properties of upper mantle around the transition zone even if the melt fraction is very low. Low velocity regions just above the mantle transition zone may result from the presence of hydrous melt. (C) 2007 Elsevier B.V. All rights reserved.