Abstract: Interleave division multiple access (IDMA) is a multiple access system with low rate codes and random interleavers, which is the only method to separate different users. Compared with code division multiple access (CDMA) systems, IDMA systems can provide more efficient scheme for multiple user detection (MUD). And IDMA can achieve the optimal combination between coding gain and multipath gain. In research area, IDMA system is looked as a special case of CDMA system and has the same channel capacity with CDMA system. But this issue is not proved from the viewpoint of information theory. Based on the code definition of IDMA, we introduce the average mutual information to measure the channel capacity. The capacity region achieved by IDMA with K users is proved with random encoding and jointly typical sequences. We prove that IDMA has the same capacity limitation with CDMA. Thus, it is proved that IDMA is a special case of CDMA from the view of information theory.