SiC/TiB_2 composites of varying TiB_2 content from 0 to 52 vol.% were prepared by pressureless sintering. When these composites were sintered at 2150°C the mechanical properties such as elastic modulus, strength and toughness increased with increasing TiB_2 content. On the other hand, at a sintering temperature of 2200°C, the mechanical properties reduced gradually with increasing TiB_2 content. The main reason was deduced from the onset of spontaneous microcracking and the critical particle size for microcracking was calculated approximately 5.6 mutextrmm.