Reactivity worth of the fuel elements of I.T.U. TRIGA MARK-II reactor are calculated by using both one-group perturbation theory and a one-dimensional, two-group diffusion computer code, TRIGAP. Results of both methods are compared with those measured experimentally. Although the perturbation theory is obtained from the diffusion theory by making some assumptions, it is seen that one-group perturbation theory gives the results with better accuracy in comparison to the results of TRICAP. This situation shows that TRIGAP cannot reflect the effect of local material changes on the multiplication factor well enough. In order to calculate fuel elements reactivity worth of TRIGA type reactors, one-group perturbation theory can be preferred to TRIGAP due to its simplicity and accuracy, when the flux and group constants are known. (C) 1998 Elsevier Science Ltd. All rights reserved.