This paper presents the phase relationships in the biotite gneiss-H20 system by melting experiment at 0.1-0.2 Gpa pressures. The solidus temperatures are 762℃ at 0.1 Gpa and 712℃ at 0.2 Gpa. The biotite-out temperatures are 787℃ at 0.1 Gpa and 737℃ at 0.2 Gpa, while the Quartz-out temperatures are 837℃ at 0.1 Gpa and 787℃ at 0.2 Gpa. Using Burnham model (1979), the estimated H2O-solubility of the biotite gneiss welt at liquidus temperature is 3.8% at 0.1 Gpa and 5.8% at 0.2 Gpa, Based on the results of the melting experiment and the. paleogeotherm of the early Precambrian continental crust in ea3tern Hebei Province, the widespread migmatization of the early Precambrian amphibolite facies rocks in the region may result mainly from the partial melting of the crustal rocks (biotite gneiss). The experiment results and the present geother in this region suggest that the crustal low-velocity layer may result from the presence of interstitial hydrous fluids rather than the partial melting.