In the present paper, a practical formula for the root bending stress of helical gears was investigated. The load sharing factor of helical gears with various helix angles, numbers of teeth and overlap ratios was calculated and, on the basis of the calculated results, an approximate equation for the load sharing factor at the worst loading position of helical gears was derived. By using approximate equations for the load sharing factor and helix angle factor, in which the effect of load distribution on the line of contact is taken into account, a new practical formula for the root bending stress of helical gears proposed. The validity of the proposed formula was confirmed by comparing the results by this formula with those by the various conventional formulas for the bending strength of helical gears and experimental results.