Abstract: In order to understand the resistance of Spodoptera litura(Fabricius) to abamectin, the susceptibility of S. litura to abamectin was studied by leaf immersion method and spectrophotometry. The results showed that S. litura from different areas had different susceptibility to abamectin in Hainan. The resistance of S. litura from Yacheng，Luodai，Jinjiang and Tongza increased 8.94–fold，4.60–fold，6.12–fold and 2.82–fold in 2004 and 24.66–fold，22.59–fold，11.24–fold and 4.41–fold in 2005 based on the results of the LC50 of S . litura from Liangyuan. The resistance of S. litura to abamectin developed very rapidly. The bioassay of carboxylesterase indicated that carboxylesterase activity in different S. litura significantly increased after treated by abamectin 18.00µg/g, 9.00µg/g, 4.50µg/g, 3.00µg/g and 2.25µg/g .The results above showed that the significant increase of carboxylesterase activity in S. litura was one of the main factors which caused the susceptibility of S. litura to abamectin to decline.