Abstract: Soil microbial biomass carbon,nitrogen and its C/N ratio are the important biological indicators characterizing soil quality. An incubation experiment was conducted to study the response of soil microbial biomass C,N and C/N ratio to three organic amendments (pig manure, rice straw and Chinese clover) in a soil contaminated by atrazine herbicide at a rate of 10 mg/kg soil. The results showed that soil microbial biomass C and N and C/N ratio were significantly decreased after the soil was contaminated by atrazine. Organic amendments applied at a rate of 10 mg/g significantly increased microbial biomass C, N and C/N ratio in atrazine-contaminated soil. The effect of organic amendments on microbial biomass C, N and C/N ratio followed the order of pig manure＞Chinese clover＞rice straw.