$SiC/TiB_2$ composites of varying $TiB_2$ content from 0 to 52 vol.% were prepared by pressureless sintering. When these composites were sintered at $2150^{circ}C$ 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 $2200^{circ}C$, 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 $mu extrm{m}$.