The C-f/SiC composites were prepared by precursor pyrolysis-hot pressing with AIN and B as sintering additives, and the effect of sintering additives on the microstructure and mechanical properties of the composites was investigated. In the composites, a SiC-AlN solid solution was formed, which was considered to be mainly due to the reactions between PCS and AIN during the pyrolysis of PCS instead of the inter-diffusion between original SiC and AIN. Although it was beneficial to the microstructure and mechanical properties of the matrix because of the formation of SiC-AlN solid solution that could strengthen the grain boundaries and suppress the coarsening of SiC grains in the matrix, AIN exhibited a small effect on the fiber/matrix interfacial bonding strength. As a result, AIN had little effect on the density and mechanical properties of the composites. In contrast, the density and mechanical properties of the composites were improved significantly by the introduction of a trace of B, and the density and strength increased with the amount of B. However, the maximum of fracture toughness was obtained in the composite with 0.5 wt.% B, which was mainly due to the desirable fiber/matrix interfacial bonding. In the case of composites with 1 wt.% B, the fracture toughness decreased in spite of the enhancement in the density and strength, which was mainly due to the strong fiber/matrix interfacial bonding. (C) 2002 Elsevier Science B.V. All rights reserved.