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Glass-forming regions were determined in the MgO-Ga2O3-B2O3 system by melt quenching method. Phase separation occured for B2O3 rich compositions. Raman spectroscopy and IR spectroscopy measurements revealed that boroxol ring groups exist in the B2O3 rich compositions and that diborate groups of 4-fold coordinated boron exist at MgO rich but Ga2O3 poor compositions. Pyroborate or orthoborate groups of 3-fold coordinated boron were found at MgO rich compositions in the ternary system. With the increase in Ga2O3 content the concentration of diborate group decreased and GaO4 tetrahedra were formed together with di-tiborate and metaborate ring groups. Hence, MgO-Ga2O3-B2O3 system as well as GaO-Ga2O3-B2O3 system forms a continuous network of 6-membered rings consisting of GaO4-tetrahedra. A possibility that Ga+3 ion might partially be incorporated in these groups was pointed out from the shifts of the Raman scattering peaks associated with Pyroborate or orthoborate groups. The smooth variation in density and refractive index of the glass with composition indicates that the structural change of glass with composition is continuous.