Abstract: An information hiding method in the process of AAC coding is proposed based on research of AAC coding standard. The proposed method modifies coefficients of the little regions by rules to embed secret information, using the features of the little data regions which generally exist in the high frequency band and have good imperceptibility. The algorithm first unpacks the cover AAC file to search for the little data region according to code books, and then gets a group of quantized coefficients by decoding a code word. Under the condition of maintaining the length of the code words, no more than one coefficient of each group is modified to embed the secret information by mapping of a pair of coefficients. Experimental results reveal that the proposed method can obtain higher hidden data capacity, without any changes of the length of AAC coding, furthermore, it has good imperceptibility and good resistance to steganalysis.