The LiNbO3 (LN)-type oxynitrides, MnTaO2N and (1 - x) MnTaO2N-xMn(4)Ta(2)O(9) were synthesized under high pressures and at high temperatures. The new LN-type oxynitride, Mn(Mn1/6Ta5/6)O2.5N0.5 (0.75MnTaO(2)N-0.25Mn(4)Ta(2)O(9)) could be obtained with trace impurities. In this compound, the displacement of B ion (Ta/Mn ion) from the center of the BX6 octahedron was smaller than that in other LN-type oxides containing Ta and/or Mn ions. This indicates that the displacement of Ta/Mn ion was reduced by doping with nitride ion.