Effect of particle size of boron carbide raw material on the phase composition, microstructure and properties of reaction bonded boron carbide composites was investigated. It was found that particle gradation can make the powder packing more compact and effectively improve the volume density of green body, decreasing the content of free Si in the composites. Addition of coarse particles can reduce the reaction between B₄C and Si, which can generate SiC phase. When the weight ratio of B₄C powders with different particle sizes (3.5, 14, 28, 45 μm) is 1.5 : 4 : 1.5 : 3, the Vickers hardness, flexure strength, fracture toughness and volume density of the composites are (29±5) GPa, (320±32) MPa, (3.9±0.2) MPa·m^1/2 and 2.51g/cm³, respectively. The retard of reaction between B₄C and Si, and the decrease of free Si content along with the shrinkage of size of Si zone in the composites, are the main reasons for the improvement of the composite mechanical properties.