In order to improve the mechanical properties of boron carbide (B4C) ceramic, a mullite-reinforced B4C matrix ceramic with complete densification was fabricated via hot pressing for the first time. The dense sintering mechanism of mullite-reinforced B4C ceramic was discussed through the phase and element analysis. A new dense sintering mechanism was found in which the diffusion of Si in mullite through the B4C matrix enhances the sintering of mullite-reinforced B4C ceramics effectively. The mechanical properties and microstructure of the composite ceramics were investigated in contrast with monolithic B4C and one kind of commercial B4C ceramic. The flexural strength and fractural toughness of B4C with 3 vol% mullite addition reached 560 MPa and 3.33 MPa.m(1/2), which is 154% and 96% higher than that of monolithic B4C, respectively.