There is a wealth of literature and test results regarding the subject on single directional gear tooth bending stress and life relationships (S-N curves)@ they have been published on various journals and handbooks over the past decades@ and several of them had been widely accepted and adapted as industry standards by different gear societies around the world. However@ very limited information regarding the bi-directional tooth bending life has been revealed. To fill in the above mentioned gap for practical usages@ the authors first intended to apply the traditional fatigue theories such as modified Goodman@ Gerber@ Morrow or similar theories with minor modifications to derive a series of S-N equations for different loading conditions@ but the correlation with the actual test results was not satisfied. Nevertheless@ from the observation on these test results@ the slopes and endurance limits on the fitted S-N curves from all the test points were reasonable closer to each other@ as long as the test gears were produced by the same material and similar manufacturing process. Based on the above observation@ the author proposes a new approach that uses the common (or averaged) slope and endurance limit@ and a series of S-N curve equations on any loading conditions can be derived@ once the single directional S-N curve has been obtained.