The effects of SiC and B4C additives in the MoSi2 matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200 degreesC by a thermal mechanical analysis (TMA). The experimental results show that the Mo2B5 reinforced phase was formed in situ in the hot-pressed MoSi2/SiC/B4C composites. Both the Mo2B5 phase and the SiC phase significantly improved the mechanical behavior of MoSi2. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi2 composite with the B4C additive. As a result, a dense and homogenous MoSi2/SiC/B4C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi2/SiC/B4C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200 degreesC in comparison with the pure MoSi2 when adding 40 vol% SiC. This MoSi2/SiC/B4C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.