Abstract: Two new methods to prepare in-situ whiskers reinforced Silicon Nitride matrix composites are proposed. When Si powders containing CaFz as catalyst are nitrided,in-situ β-Si₃N₄ whiskers grow in the reaction bonded silicon nitride. In-situ β-SiC whiskers grow on the Si₃N₄ matrix and in the pores,after the porous compacts of Si₃N₄ are infiltrated with phenolic resin and tetraetyl silicate,La₂0₃ as catalyst. There is a droplet on the top of each β-SiC whisker,which suggests that it grows by the Vapour-Liquid-Solid mechanism. When two and more such whiskers with different directions occur,the whisker network is formed. The microstructure of in-situ whisker depends on the experimental conditions. With the time and temperature increased,the in-situ whiskers become wider and longer. The experimental results show that in-situ whiskers can increase not only the fracture strength but also fracture toughness.