A two-dimensional, two-group perturbation theory calculation method has been proposed for the rapid and accurate calculation of the reactivity worth for. TRIGA core fuel elements. The method requires shorter computation times and provides almost the same level of accuracy as a conventional direct eigenvalue calculation method using the CITATION code. Values for changes in macroscopic cross-sections and diffusion coefficients are obtained from cell calculation The reactivity worth for each individual fuel element is calculated on the basis of the above-mentioned changes, by means of the two-group, first older perturbation method using normal and adjoint neutron fluxes calculated beforehand for a normal core. The method was applied to a 250 kW TRIGA Mark-II reactor. The results of the calculations are in agreement with the experimental values for fuel elements in the locations C3, D8, E7 and F4. The reactivity worth for the TRIGA core fuel element placed in location B2 calculated by means of this method is approximately 5% greater than that calculated by means of the CITATION code which is based on a conventional direct eigenvalue calculation method.