The purpose of this research work was to investigate the larvicidal activities of Aster 20SP against Culex pipiens and biological effect of this compound. The larvicidal activities of Aster 20SP were tested against the third and fourth instar larvae of Culex pipiens, the mortality was observed after 24h post the treatment. The data were subjected to probit analysis to determine the lethal concentration (LC50 and LC90). The obtained results show a high mortality with the increase in the concentration, at third stage, larval mortality increased from 18.23 % at 0.7 mg /L to 45.16 % at 3mg /L in direct effect. The LC50 and LC90 were measured as 3.61 and 5.16 mg/L respectively. Cumulate mortality increased from 35.30 to 54.34% at 0.7mg/L and 3mg/L respectively. The LC50 and LC90 were 3.38 and 4.68mg/L respectively. At fourth stage, larval mortality increased from 5.50 % at 0.7 mg /L to 19.29 % at 3 mg /L in direct effect, the LC50 and LC90 was 4.34 and 6.52 mg/L respectively. In indirect effect, the mortality increased from 9.16% to 30.80% at 0.7mg/L and 3mg/L respectively, the LC50 and LC90 was 4 and 5.87 mg/L respectively. The results indicated the larvae duration increased with the increased in the concentration. In contrast, in adult stage, the results indicated high reduction in fecundity.It was concluded from this study that Aster 20SP as a potential pesticide to control the larvae stage of mosquito Culex pipiens.