We carried out Digital rock experiments using the lattice gas automation (LGA) method to investigate the relationship between resistivity index and water (oil) saturation (I-Sw) of reservoir rocks. The effects of irreducible water are examined with LGA simulation by adjusting the height of the irreducible water film. Mechanisms that control I-Sw relationship at micro pore scale are studied with oil and water saturated digital rock samples. An algorithm has been developed to construct the digital rock samples through pileup of matrix grains. The LGA was then applied to study the effects of pore geometry and fluid distribution on the flow of electrical current through the pores of these saturated digital rocks to reveal the nature of non-Archie relation of I-Sw in reservoir rocks. Based on these experiments, we developed a new equation for the of I-Sw relationship to improve the calculation of pore fluid saturation. The calculated results with this new equation have shown better fit to the laboratory measurements.