Crystallized Lu–Si–O–N phases were believed to be the grain-boundary (GB) phases that might provide Si3N4 with excellent high-temperature mechanical properties. However, little is known about the intrinsic properties, as well as the synthesis, of the Lu–Si–O–N ceramics. This work reveals the reaction paths of heating Lu2O3, SiO2, and Si3N4 powder mixtures (with the stoichiometry of 4:0.96:1) from room temperature to 1600°C. Thereafter, dense Lu4Si2O7N2 samples are synthesized by in situ reaction/hot-pressing method, and the mechanical properties at room temperature and elevated temperatures are reported for the first time. The Lu4Si2O7N2 samples show significant high-temperature mechanical properties, such as the elastic stiffness remains 77% from room temperature to 1500°C; and bending strength keeps 93% from room temperature to 1400°C. The present results shine a light on Lu4Si2O7N2 as a promising target GB phase for the optimization of high-temperature mechanical properties of Si3N4.