The friction performance and thermal resistance of paper-based wet friction materials used for automotive automatic transmissions are influenced by the material structure formed as a result of the combination of porosity and resiliency. In our previous papers, the effects of porosity on the frictional performance and the thermal resistance were characterized by Stribeck Diagrams based on continuous sliding test data, and were also discussed with reference to torque curves and temperature measurements by an SAE No. 2 friction tester. In this paper, the effects of porosity on the mechanical strength and the fatigue of paper-based friction materials were studied for practical paper-based materials. The mechanical strength like shear strength, etc. of the material tends to decrease as the porosity increases. The fatigue life of the material, when the repeated compression and shear force are applied at the same time, was also discussed.Significant insights were obtained from the experimental data of this study, which may be applied in the design of paper-based friction materials.