B₄C, Si, Al, and γ-Al₂O₃ were used as raw materials, formed β-Sialon and SiC in situ by solid-state reaction for preparation of B₄C composite powder. β-Sialon and SiC grains are uniformly and dispersedly distributed in the composite powder. The effects of the molar ratio of raw materials, nitridation temperature and soaking time for the phase composition, and microstructure of the composite powder were studied. When the molar ratio of B₄C and Si is 2: 3, the composite powder with columnar β-Sialon grains growing alternately can be prepared after nitridation at 1500°C for 4 h. The liquid phase formed by the melting of Si at high temperature promoted the dissolution of B4C and the formation of SiC. When the nitridation temperature was increased or the soaking time was prolonged, the gas phase volatilization of Si and Al was intensified, SiC and AlN whiskers tend to be formed in the composite powder.