The tomographic gamma scanning (TGS) system is best suited for using a sodium iodide (NaI) detector due to its low cost and ability to operate at room temperature. The appropriate collimator design of the TGS system reduces cross interference between the peaks and improves detection efficiency. A collimator design approach has been suggested and combined with the Monte Carlo method to optimize the collimator for a TGS system with a NaI detector. The experimental work, which was performed using standard nuclear material, showed that this method could be used to produce a collimator compatible with the NaI detector in the TGS system.