Abstract: This paper is the synthesis of a durability test on about hundred pairs of various grade of spheroidal graphite iron gears. By mathematical processing, S-N curves for these irons and their endurance limit values vH1jmσ(Hlin) andσ(Flim). Corresponding to 50%and 1%failure probability are obtained. The test resultsσ(lin) andσ(lim) values of soft-nitrided ferritic and baintitic spheroidal graphite iron gears shall make valuable supplements to endurance data of ISO/TC60WG6 200E and 201E.From these test results analysed, authors have made valuations for properties of these spheroidal graphite iron gears.