Abstract: To assess the safety of lambda-cyhalothrin used in paddy field to aquatic organisms, the acute toxicity of five different formulations(EC,ME,EW,WP and SC) of lambda-cyhalothrin to main aquatic organisms were determined by semi-static state method. The fate and risk of lambda-cyhalothrin to the aquatic organisms in paddy field-pond mimic ecosystem were also studied. The results showed that lambda-cyhalothrin is highly toxic to both fish and shrimp, especially to shrimp. The toxicity of five formulations of lambda - cyhalothrin to the same aquatic organisms were different. The lambda - cyhalothrin was degraded quickly in the environment, the half lives of different formulations of lambda- cyhalothrin in paddy field water, pond water, and paddy field soil ranged from 0.23 d to 0.53 d, 0.38 d to 0.63 d,and 0.96 d to 7.35 d, respectively. The water overflow from paddy field since raining after 12 h of the use of lambda-cyhalothrin was safe to fish, mussel and crab, but was seriously hazardous to shrimp. However, the water overflow was safe to all the aquatic organisms tested after 4 d of use of all formulations.