Abstract: A cost effective SiC/Si₃N₄ nano composite was obtained by in situ synthesis of precursor, using polycarbosilane as precursor. The results showed that the ambitious flexural strength and fracture toughness of this material had improved. The formation of this composites microstructure and the mechanism of fracture were discussed at the same time.It was important to control the formation of SiC nano crystals and the growth of rod like β Si₃N₄ grain.The formation of the intragranular structure and the rod like β Si₃N₄ grains(aspect ratio more than 7.5) was the reason for the improvement properties.