This paper first presents an impossible differential property for 5-round Advanced Encryption Standard (AES) with high probability. Based on the property and the impossible differential cryptanalytic method for the 5-round AES, a new method is proposed for cryptanalyzing the 8-round AES-192 and AES-256. This attack on the reduced 8-round AES-192 demands 2~ 121 words of memory, and performs 2~ 148 8-round AES-192 encryptions. This attack on the reduced 8-round AES-256 demands 2~ 153 words of memory, and performs 2~ 180 8-round AES-256 encryptions. Furthermore, both AES-192 and AES-256 require about 2~ 98 chosen plaintexts for this attack, and have the same probability that is only 2~ -3 to fail to recover the secret key. Based on the property and the imposition differential cryptanalytic method for the 5-round AES, a new method is proposed for cryptanalyzing the 8-round AES-192 and AES-256. This attack on the reduced 8-round AES-192 demands 2-121 words of memory, and performs 2-148 8-round AES- 192 encryptions. 256 require 2 to 153 words of memory, and perform 2 to 180 8-round AES-256 encryptions. Furthermore, both AES-192 and AES-256 require about 2 ~ 98 chosen plaintexts for this attack, and have the same probability that is only 2 ~ -3 to fail to recover the secret key.