Abstract: The root stress of model JB2940-81 double circle-arc gears which have beer, sufficiently run-in were measured by micro strain gauges on a locked-in torque test stand with.center distance of 120m m. Both the relationships between stress and the helix angles,and between stress and external load were measured respectively. After analyzing the measured data a stress formula was derived which may be served as a fundamental equation of root strength for double circle-arc gears.During the meshing drives the influence of the neighboring tooth load on maximum root stress of gauged tooth and the influence of two contact spots of gauged tooth on, maximum root stress which are reinforced have been measured. These rules may be served as a reference for designing new tooth profile and for choosing parameters of gear pair.