Reaction-bonded B4C–SiC composites are highly promising materials for numerous advanced technological applications. However, their microstructure evolution mechanism remains unclear. Herein, B4C–SiC composites were fabricated through the Si-melt infiltration process. The influences of the sintering time and the B4C content on the mechanical properties, microstructure, and phase evolution were investigated. X-ray diffraction results showed the presence of SiC, boron silicon, boron silicon carbide, and boron carbide. Scanning electron microscopy results showed that with the increase in the boron carbide content, the Si content decreased and the unreacted B4C amount increased when the sintering temperature reached 1650°C and the sintering time reached 1 h. The unreacted B4C diminished with increasing sintering time and temperature when B4C content was lower than 35wt%. Further microstructure analysis showed a transition area between B4C and Si, with the C content marginally higher than in the Si area. This indicates that after the silicon infiltration, the diffusion mechanism was the primary sintering mechanism of the composites. As the diffusion process progressed, the hardness increased. The maximum values of the Vickers hardness, flexural strength, and fracture toughness of the reaction-bonded B4C–SiC ceramic composite with 12wt% B4C content sintered at 1600°C for 0.5 h were about HV 2400, 330 MPa, and 5.2 MPa·m0.5, respectively.