A series of h-BN based composites with Yb 4Si 2O 7N 2 as a secondary phase were successfully synthesized by an in situ reaction hot pressing method. It was found that the relative density and room-temperature mechanical properties monotonically increased with increasing the content of Yb 4Si 2O 7N 2 from 20 to 50 vol%. When 50 vol% Yb 4Si 2O 7N 2 was introduced, the relative density of the composite reached 98.75%, and its flexural strength, compressive strength, fracture toughness, and hardness reached 338±10 MPa, 803±49 MPa, 2.06±0.06 MPa·m 1/2, and 2.69±0.10 GPa, respectively. The strengthening effect of Yb 4Si 2O 7N 2 was mainly attributed to its high modulus and high hardness. Fine microstructure was also advantageous to strength and could lead to more tortuous crack propagation paths and then improve the fracture toughness of the composites simultaneously. Meanwhile, the composites maintained good machinability.