Three dimensional numerical analysis of fatigue crack initiation and the analysis of maximum principal stress were performed at the loaded side of the gear tooth root. Thin-rimmed cylindrical involute gears with external teeth were analyzed. Effects of the rim thickness, web arrangement, applied torque and rotational speed related to centrifugal force were evaluated. Elastic FEM analysis was used to determine the magnitude and location of maximum principal stress. The influence of load distribution on the face width is taken into account through the modeling of contact between teeth in mesh. The numerical model was validated experimentally using strain gauges. Subsequently, the model was used to determine the stress history at the gear tooth root. We have taken into account the fact that in actual gear operation the magnitude and the position of load change as the gear rotates through the mesh, causing a nonproportional stress state. On the basis of known cyclic material properties, the time to fatigue crack initiation and the potential location of a fatigue crack were determined by numerical computation using a combination of the local strain-life and the critical plane method. The obtained results were compared with other investigators’ numerical and experimental results, and thus validated. When load is applied at the outer point of single-tooth contact, it was shown that the magnitude and the location of the maximum principal stress in the tooth root, depend on the rim thickness, web arrangement, applied torque and centrifugal force. The obtained results deviate from those obtained by conventional calculation. Potential fatigue crack initiation is located away from the location of maximum principal stress. In the case of the analyzed gear material, steel C45E, it depends on the rim thickness and web arrangement. The gear service life, estimated in regard to fatigue crack initiation, depends on the applied torque, rim thickness, and web arrangement, while the influence of centrifugal force is insignificant for rotational speeds of common gear drives. A non-conservative mistake in the determination of gear service life is made by assuming that the initial crack occurs at the location of maximum principal stress. The allowable stress number at tooth root for thin-rimmed gears is different from that for solid gears, and depends on the rim thickness and web arrangement. Service life of gears and the bending strength of toot root can be more accurately and reliably estimated using the numerical model and the results presented in this work.