Abstract: Chilo suppressalis（Walker）is is one of main pests of rice, and recently its harm to rice is increasingly serious in parts of China. C. suppressalis is difficult to classify its larval instar, and the scientific standard for instar classification is lack. Therefore, it’s necessary to further clarify instars classification standard of C. suppressalis larvae, and to provide a scientific basis for the prediction and control of C. suppressalis. The experiment insect source was got from the the larvae population of C. suppressalis raised by rice seedlings, and the overwintering larvae of C. suppressalis were collected from rice piles in fields.Using Environmental Scanning Electron Microscope（ESEM）techniques, the varied characteristics of number of crochets on prolegs and head width of C. suppressalis larvae was studied systematically to develop a scientific the standard of discriminative of diferent instar larvae. The average number of crochets on prolegs of C. suppressalis larvae from the 1^st to 6^th instars was 6.2, 13.0, 18.4, 30.0, 41.0 and 49.2 respectively, while the average number of crochets on prolegs of the overwintering larvae was 41.0. At the same time, the number of crochets on prolegs of C. suppressalis larvae increased significantly with the elder instar. And there were significant differences in the number of crochets on prolegs among different instar larvae. But the number of crochets on prolegs of overwintering larvae was not significantly different from that of the 5^th instar larvae. The head width of C. suppressalis larvae（1st-6th instar）was broadened as the instar of the larva increased, which was 0.25 mm, 0.45 mm, 0.68 mm, 0.98 mm, 1.15 mm and 1.31 mm respectively, while the head width of overwintering larvaewas 1.44 mm. The head widths of different instar C. suppressalis larvae were different significantly, and the overwintering larvae of C. suppressalis had the longest head width. The number of crochets on prolegs and head width of the C. suppressalis larvae can accurately distinguish different instar laraveand overwintering larvae of C. suppressalis. Moreover, this study also indicates the the classification criteria for different instar and overwintering larvae of C. suppressalis, according to the number of crochets on prolegs and head width.