Two-phase materials have been assumed to contain spherical particles arranged in a three-dimensional cubic geometry. The arrangement has been divided into unit cells each of which contains a sphere. The resistor model has been applied to determine the effective thermal conductivity (ETC) of the unit cell. A sphericity term has been introduced to account for the ETC of materials with non-spherical particles. A porosity correction term has also been applied to the present model and other models cited in the literature. Reported results have been discussed in the light of the proposed model and suggested modifications.