Silicon carbide whisker (SiC w ) toughened B 4 C composites have been prepared by pressureless infiltration of B 4 C–SiC w –C preforms with molten silicon under vacuum at 1500°C. The effect of SiC w addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiC w /B 4 C composites is discussed. It is revealed that the addition of SiC w improves the fracture toughness of B 4 C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiC w /B 4 C composite with 24wt% SiC w addition is 4.88MPam 1/2 , which is about 9% higher than that of the one without SiC w , but at the same time, the bending strength reduces to the minimum value 243MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiC w /B 4 C composites is B 4 C, SiC, Si, and B 12 (C, Si, B) 3 , with no residual C. And the main toughening mechanism of SiC w is whisker pulling up.