Traditionally, densification and grain growth are two competing processes in sintering of ceramics. To improve the density, while limiting grain growth at the same time, an ultrahigh pressure (1 GPa) is employed here and results in plastic deformation as the dominant densification mechanism during the sintering process. In this way, fully dense boron carbide (B4C) structural ceramics without grain growth is prepared under the pressure of 4.5 GPa at low temperature of 1300 degrees C in 5 minutes, while showing excellent mechanical properties such as Vickers hardness of 38.04 GPa, Young's modulus of 487.7 GPa, and fracture toughness of 3.87 MPam(1/2). This study should also facilitate the development of other structural ceramics for practical applications.