This paper presents a study on effects of rim and web thicknesses on root stresses of thin-rimmed helical gears. Root stresses of thin-rimmed helical gears with symmetric web arrangement of various rim and web thicknesses, which were meshed with the solid helical gear, were measured from the beginning of engagement to the end of the engagement by the strain gauge method. The changes of the root stress from the beginning of engagement to the end of engagement were examined for the thin-rimmed helical gear with symmetric web arrangement. The meshing position (worst loading position), where the maximum root stress occurred, was determined for the thin-rimmed helical gear with symmetric web arrangement. The effects of rim and web thicknesses on the root stress, the maximum root stress and the position of the maximum root stress were clarified.