The variation of the porosity correction factor, β, with temperature and pore shape was studied by using the Fricke equation for the thermal conductivity of a two-phase medium containing the second phase as randomly distributed ellipsoids. The temperature variation occurs via a parameter, γ, related to the conductivity of the pores. The effect of pore shape was determined via the axial ratio, ε, of oblate or prolate ellipsoidal pores. The results are presented graphically in curves showing the variation of β with γ and ε. The model was applied to actual conductivity data of ThO 2 and ThO 2-UO 2 specimens in an effort to correlate the general porous structure typical of ceramic fuels to the parameters (ε and γ) in the Fricke equation. It was shown that the β factor for ceramic fuels can be related to their general microstructure and pore conductivity in a self-consistent mathematical framework. The technique, therefore, minimizes much of the guesswork heretofore required in estimating the porosity correction factor.