Abstract: The effects of Silwet 408 on surface tension and contact angle of abamectin 3% microemulsion(ME) on apple leaves, as well as that on toxicity of abamectin 3% ME against Panonychus ulmi and Aphis citricola were studied by indoor experiments and field trials. The results showed that adding Silwet 408 could reduce effectively the surface tension and static contact angle of abamectin 3% ME on apple leaves. Moreover, the surface tension and static contact angle decreased gradually as the content of Silwet 408 increased from 3% to 10%. The results of bioassay showed that adding Silwet 408 into abamectin 3% ME could enhance significantly the toxicity and control efficacy to P.ulmi, but not to A.citricola. The results suggested that when adding adjuvant into pesticide for synergistic purpose, the pest species should also be considered besides the surface tension and contact angle.