High-temperature properties of silicon nitride ceramics with Lu-silicon-oxynitride grain boundary phases were investigated. Si3N4 powder with 1.2 mol% (SN12) and 4.8 mol% (SN48) of Lu2O3 were gas-pressure hot-pressed at 1950°C for 2 h under 20 MPa in 1 MPa N2. SN12 consisted of elongated β-Si3N4 and a secondary phase, Lu4Si2O7N2, whilst SN48 consisted of elongated β-Si3N4 and Lu4Si2O7N2 + Lu2SiO5. At 1500°C and 1600°C, the stress-strain curve of SN48 was nonlinear, whilst that of SN12 was linear, indicating that SN12 broke as a brittle fracture at these temperatures. SN12 had excellent oxidation resistance and weight gain during the oxidation at 1500°C for 1000 h was 4 g/m2. Creep lifetime of SN12 at 1500°C under tensile stress of 137 MPa exceeded 1678.5 h.