We report on the method for monitoring of the modifications in Yb-doped optical fibers irradiated by gammarays with the doses 1, 2, 4 and 8 kGy. This method is based on the refractive index change (RIC) effect induced in the tested fibers by square modulated optical pump pulses of 1 ms duration at 980 nm with the power up to 100 mW. The applied in-situ monitoring is based on interferometric technique and highlights the key parameters of the RIC effect that allows to characterize quantitatively the degree of the fiber degradation under the gamma irradiation.