A detailed study of the d. c. electrical conductivity and the optical absorption spectra as a function of gamma-irradiation doses for prepared (20 mole% Nb2O5 -80 mole% P2O5) glass system is presented. The temperature dependence on the d. c. electrical conductivity measurements is studied over the temperature range from room temperature to 573 K. The induced changes in the d. c. electrical conductivity caused by different doses of gamma irradiation (0-16 Mrad) were studied. The electrical conductivity, sigma, and the activation energy, E, values were found to be sensitive to the gamma-ray doses. The optical absorption spectra were measured in the wavelength range from 200 to 1100 nm at different gamma -doses, in the range from 0 to 28 Mrad. The obtained results are analysed assuming optical absorption by indirect transition. Values of the absorption coefficient, alpha, the optical energy gap, E-opt, and the width of the band tail, DeltaE, are found to be gamma -irradiation dose dependent.