An experimental study was performed on the melting and subsolidus phase relations in a synthetic magnesium silicate system at a pressure of about 20 GPa and over the temperature range of 1700~2500¿ C under hydrous conditions. The modified spinel, Mg2SiO4, is stable up to 2250¿ C, above which is melts incongruently into phase B, Mg23Si8O42H6, and liquid. At higher temepratures phase B melts again incongruently into periclase and liquid. The liquid coexisting with periclase has an atomic ratio Mg/Si~1.5 congruently into periclase and liquid. The liquid coexisting with periclase has an atomic ratio Mg/Si~1.5 at 2360¿ C and ~1.7 at 2500¿ C. For MgSiO3 composition the ilmenite is stable at lower temperatures, and it transforms to a garnet with increasing temperatures to 2000¿ C. At above 2200¿ C, clinopyroxene replaces the garnet and then melts congruently at 2250¿ C. The triple point of clinopyroxene, garnet, and liquid is closely located at 20 GPa and 2250¿ C. The binary system of Mg2SiO4-MgSiO3 shows a eutectic melting between modified spinel and garnet with a eutectic point located at 2150¿ C and Mg/Si~1.3.