Abstract: The resistance of alphamethrin in Myzus persicae (green peach aphid) was selected in laboratory. The resistance development was monitored through the bioassay and the 96-well microplate reader for measuring change of esterase (Est). The results showed that the resistance level gradually rose with the increase of selection time. After three generations,the resistance of green peach aphid was 13-fold. While ten generations later,the resistance amounted to 49.86-fold. It suggested that green peach aphid developed obvious resistance to alphamethrin. As resistance developed,enzyme protein content increased significantly from 35.8632 to 66.2990 μg/mL with Est activity varying from 33.6760 to 171.0052 μmol/(mL·min). Comparing the frequency distribution of enzyme protein content and activity,we found that more and more individuals possessed higher content and activity. And there is a significant correlation between LD50 and enzyme protein content,Est activity respectively. The results showed that the Est played an important role in resistance development of green peach aphid to alphamethrin.