The root stresses of thin rim spur gears are investigated by means of the two-dimensiona1 finite element method (FEM) with triangular elements. The effects of boundary constraints for gear models and of adjacent teeth on the root stress distribution are discussed. The gear tooth model with a single tooth is shown to be available for a sufficiently precise evaluation of root stresses. A suitable rim thickness of gear model for the stress analysis by FEM is proposed. The true root stresses are computed and measured for gears of different rim thicknesses. The true stresses computed by FEM are in a fairly good agreement with measured ones. On the basis of these investigations the effects of rim thickness on the root stresses and on the critical sections in thin rim spur gears are also discussed.