The effects of microstructure on strength, toughness and corrosion resistance of reactionformed silicon carbide and heat treatment on the properties of reaction-formed silicon carbide have been investigated in this paper. The results show strength and corrosion resistance of reaction-formed silicon carbide decrease with increasing Si content. Fracture toughness shows no rule with free Si content. The average SiC grain size and bonding area of SiC grain increase during heat treatment at 1850°C. The strengths of the samples after heat treatment decrease compared with that of the samples before heat treatment due to the pores in the heat treatment bodies.