This paper presents a study on the effects of design parameters (rim thicMness, standard pressure angle, number of teeth, and face width) on root stresses of a meshed thin-rimmed helical internal gear with an asymmetric web arrangement for a planetary gear unit of the automatic transmission of automobile. The tooth deflections of the thin-rimmed helical internal gear with an asymmetric web arrangement of various design parameters due to a concentrated load were calculated by the three-dimensional finite-element method (3D-FEM). On the basis of the calculated results, approximate equations for the influence function of the tooth deflection were derived. The load sharing factor and the load distribution on the contact line of the thin-rimmed helical internal gears in mesh were calculated using these approximate equations and Kubo and Umezawa's method. The root stress of the thin-rimmed helical internal gear under the distributed load was calculated using the 3D-FEM. and the effect of the design parameters on the root stress was determined. Furthermore, a practical formula for the maximum root stress of the thin-rimmed helical internal gear with asymmetric web arrangement in mesh was derived.