Phase relationships in the Si3N4-SiO2-Lu2O3 system were investigated at 1850degreesC in 1 MPa N-2. Only J-phase, Lu4Si2O7N2 (monoclinic, space group P-2/c, a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and beta = 109.793(6)degrees) exists as a lutetium silicon oxynitride phase in the Si3N4-SiO2-Lu2O3 system. The Si3N4/Lu2O3 ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu-J-phase, beta-Si3N4, and a new phase of Lu3Si5ON9, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) run, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si3N4-LuN-Lu2O3 system. The phase diagram suggests that Lu4Si2O7N2 is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.