Abstract: The phase diagram of the binary system MnO-Nb_2O_5 was studied by means of the differential thermal analysis(DTA),X-ray diffraction analysis and scanning electron microscopy(SEM).Thermodynamic da- ta of the new compounds 4MnO·Nb_2O_5 and MnO·Nb_2O_5were found as follows: 4MnO·Nb_2O_5:t_m=1398±2℃ △H_m=129000(J/mol) △S_m=77(J/mol,K) △G_m~o=129000-77T(J/mol) MnO·Nb_2O_5:t_m=1499±2℃ △H_m=86940(J/mol) △S_m=49.6(J/mol,K) △G_m~o=86940-49.6T(J/mol) The eutectic parameters for the system are given below: t_(E1)=1383±3℃ N_(E1(MnO))=0.896 t_(E2)=1312±2℃ N_(E2(MoO))=0.713 t_(E3)=1400±4℃ N_(E3(MnO))=0.231

Key words: phase diagram, MnO-Nb_2O_5, system, DTA