SiC-BN composites were prepared via the proposed in situ reaction, which used Si3N4, B4C, and carbon as reactants. Adding SiC powder to the reactants controlled the BN content in the composite. For comparison, SiC-BN composites with the same phase compositions were produced via conventional processing. The in situ process was advantageous for obtaining better composites with fine grain size and homogeneous microstructures. The in situ composite that had a BN content of 53.71 vol% exhibited considerably high strength (342 MPa) and a very low elastic modulus (107 GPa). The SiC-25-vol%-BN in situ composite had a peak strength of 588 MPa, which was 95% of that of monolithic SiC; however, the elastic modulus was as low as half that of monolithic SiC. These irt situ SiC-BN composites can be expected to have excellent thermal shock resistance and mechanical strain tolerance.