Reaction-bonded B4C-SiC composites are highly promising materials for many advanced technological applications. However, their microstructure evolution mechanism remains unclear. Herein, B4C-SiC composites were fabricated by the Si melt infiltration process. The influence of sintering time and 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 increasing of boron carbide addition, the amount of Si content decreased and the amount of unreacted B4C increased. Unreacted B4C diminished with increasing sintering time and temperature. The further microstructure analysis showed a transition area between B4C and Si, with a C concentration marginally higher in the transition area than in the Si area. It indicates that after the silicon infiltration，diffusion mechanism is the primary sintering mechanism of the composites. As the diffusion process progresses, the hardness increases. 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℃ for 0.5 h are 2600 HV, 330 MPa, and 5.2 MPa·m0.5, respectively.