The morphological stability of hydrous liquid droplets at grain boundaries of eclogite minerals in the basaltic part of a subducting slab were investigated based on experimental constraints for the dihedral angles. We measured the dihedral angles in the eclogite–H2O system at a temperature of 1000 °C and at pressures ranging from 4 to 16 GPa. The dihedral angle of hydrous liquid versus garnet–garnet (θGG–L) increased with increasing pressure from 46° at 4 GPa to 66° at 12 GPa, although it showed a weak decreasing trend at pressures higher than 12 GPa. The dihedral angle of hydrous liquid versus clinopyroxene–clinopyroxene (θCC–L) was almost constant with increasing pressure (61 and 59° at 4 and 10 GPa, respectively). The dihedral angle of hydrous liquid versus garnet–clinopyroxene (θGC–L) was 73–76° at 4–10 GPa, and it was always higher than θGG–L and θCC–L. By applying the morphological stability criteria for liquid in a system with two solid phases (garnet and clinopyroxene) and bond percolation theory for a three–dimensional lattice of tetracoordination, we found that the hydrous liquid was isolated at the grain edges and corners of eclogite minerals in the cold slab under a wide range of pressure conditions of the upper mantle from 4–14 GPa when the grain size of garnet was equal to that of clinopyroxene. Thus, basaltic crust containing hydrous liquid droplets may carry water to the lowermost upper mantle and the mantle transition zone when the slab is cold.