Bending fatigue is one of the main failure modes of gears. The method of gear fatigue design was discussed in many papers. Most of them are based on GB/T3480-83.But study shows that the method current widely used only fits for the single side loaded gear. For double sides loaded gear, only a mean stress modification coefficient is concerned compared to single side loaded gear. A new formula based on fatigue theory is given to modify the mean stress in consideration of the load character of doubled sides loaded gear. The method given in the paper is an addition to the traditional gear design method.