To apply the approximate calculation formulae for tooth fillet and root stresses of a thin-rimmed gear (3) to the case of an internal gear, it is necessary to obtain the nominal stresses of rim. A general method to calculate the nominal stresses of rim of an internal gear has been shown for the case of an internal spur gear which is fixed or supported by different methods, e.g., for the case of bolt fixing and/or geared coupling supporting of the internal gear. The tooth fillet and root stresses obtained by this calculation method for an internal gear which is fixed bolts have shown fairly good agreement with the results of the FEM calculations and those of the stress measurements. Using this simple calculation method, influences of rim thickness and umber of teeth in the internal gear and number of fixing bolts on the stress state at fillet and root areas have been illustrated.