The austenite grain size in forgings and rolled stock used for the production of medium-size and large nitrided toothed gears is No. 5–7, depending upon the grade of the steel. Practical experience has shown that it is necessary to obtain a grain size no greater than No. 8 in the billets in order to achieve the higher serviceability in gears of this type. No information is available concerning the utility of fisrther reducing the grain size of nitriding steels. In view of this, the effect of austenite grain size on the structure and contact durability of nitrided steel was evaluated.