Four Mo/Mo 5 SiB 2 in-situ composites with different volume fractions of Mo solid solution, V f M o , were produced by ingot metallurgy and their microstructure, thermal expansion, compressive strength and oxidation behavior were investigated relating to V f M o . Mo-8.5Si-13.2B annealed at 2073 K for 24 h has nearly eutectic microstructure consisting of Mo solid solution and Mo 5 SiB 2 . For higher V f M o , there are micro-cracks in Mo 5 SiB 2 phase probably induced by thermal stress during the cooling process. Mo-8.5Si-13.2B shows the highest yield stress among the four Mo/Mo 5 SiB 2 in-situ composites, and about 1 GPa even at 1773 K. For lower V f M o than that of Mo-8.5Si-13.2B, the steady state oxidation behavior is observed at temperatures between 1173 and 1473 K, indicating that the oxidation resistance is relatively good, whereas the oxidation resistance is catastrophic for higher V f M o than that of Mo-8.5Si-13.2B.