Abstract: Acyrthosiphon pisum is one of the most devastating pests of alfalfa (Medicago sativa L.) in China. Using micro-satellite markers, we studied the genetic similarity and genetic differentiation among the pest geo-populations, and their relationships with genetic distance, geographical distance and elevation. We also evaluated genetic structure of ten A. pisum (green form) populations in different regions of China (Beijing City, Inner Mongolia Autonomous Region, Ningxia Hui Autonomous Region, Xinjiang Uygur Autonomous Region, and Gansu, Shaanxi, Shandong, Henan, Shanxi and Qinghai Provinces). A total of 41 (100%) polymorphic bands were detected using 15 pairs of SSR primers in 300 individuals across 10 populations. The number of observed alleles (N_a) per locus was 1.592 7 and that of effective alleles (N_e) was 1.356 9, with Nei's genetic diversity (H) of 0.206 6 and Shannon-Wiener diversity index (I) of 0.307 6. Populations from Xinjiang Uygur Autonomous Region, Shaanxi Province and Henan Province showed strong genetic diversity. In contrast, genetic diversity of Mongolia Autonomous Region, Shandong Province and Qinghai Province populations was much weak. Overall, there was a high genetic divergence among the 10 A. pisum populations (G_st = 0.399 6). Based on cluster analysis, the 10 A. pisum populations were divided into two distinct clusters. Cluster one (i) consisted of the populations from Shandong and Henan Provinces, while cluster two (ii) consisted of the populations from Beijing City, Inner Mongolia Autonomous Region, Ningxia Hui Autonomous Region, Xinjiang Uygur Autonomous Region, and Gansu, Shaanxi, Shanxi and Qinghai Provinces. Mantel-test analysis showed that genetic distance of A. pisum was no significantly correlated with geographic distance and elevation. As A. pisum (green form) population showed relatively high genetic diversity in China, there was the need to strengthen monitoring and management of pea aphids.