Abstract: The security of a class of selective encryption scheme was analyzed. According to the characteristics of the correlation of adjacent image pixels, an algorithm to attack the encryption scheme was given using the disclosed information from cipher text and the method of known-plaintext attack, and the validity of the algorithm was verified by experiments. The experimental results showed that the higher the correlation of adjacent pixels of an image, the less the plaintext available to recover more information in plaintext by use of not encrypted information in cipher text. Therefore, for an image which has a high correlation between adjacent pixels, the selective encryption scheme is unsecured. The selective encryption scheme is thus correspondingly.