Boron carbide ceramics were simultaneously synthesised and densified from B and C elemental powders by a spark plasma sintering method. The reactive sintering process was investigated, and two stages of large shrinkage corresponding to synthesis and densification of boron carbide were observed. At stage I (1300–1700°C), B reacted with C and the product in phase of free C and B rich boron carbide (B4+xC) was synthesised. With increasing the sintering temperature, C atoms diffused into the B4+xC lattice, resulting in the reduction in free C content and the decreasing in lattice parameters of B4+xC. At stage II from 1700 to 1900°C, the densification of the synthesised boron carbide occurred. The product was densified gradually and a high relative density (95%) was obtained by spark plasma sintering at 1900°C.