Since the channel estimation performance is affected directly by the positions of pilot tones, the optimization of pilot placement process known as having quite important role on minimizing the estimation errors has become a significant task for the multicarrier transmission technologies. Therefore, we propose Grey Wolf Optimizer (GWO) for pilot tones optimization to enhance the performance of Least Squares (LS) algorithm employed for channel estimation in Orthogonal Frequency Division Multiplexing - Interleave Division Multiple Access (OFDM-IDMA) system. Besides, the upper bound of Mean Square Error (MSE) which can be derived through Gershgorin disc theorem is used as an objective function of GWO to get rid of computing matrix inversion that is indispensable when computing MSE. In the simulations, our proposed GWO based pilot design method is compared to random, equispaced, Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) based pilot placement strategies in terms of MSE and bit error rate (BER) criteria. The simulation results in which our proposed technique leaves behind the other considered strategies demonstrates that GWO based pilot design method is a promising pilot placement strategy for OFDM-IDMA system because of its high performance and low computational complexity. (C) 2017 Elsevier B.V. All rights reserved.