Here, we propose a novel tree based interleaver (TBI) for generating user specific chip-level interleaving sequences for various users in an IDMA system, which reduces computational complexity drastically. This interleaver also solves the memory cost problem and reduces the amount of information exchange between mobile stations and base stations required to specify the interleaver. Simulation results are presented to show that the proposed TBI perform well as compared to master random and random interleavers in an IDMA scheme.