Abstract: Reports the experimental investigation on the reliability of gear-tooth bending fatigue strength for 20CrNi2MoA steel gear of carburization-quench hardening.The tests were carried out on a electro-magnetic resonance fatigue test machine of type STRON 1603(U.K). The grouping test method at four constant stress levels was applied for short life-span areas while the stress up-and-down test method were used for long life-span areas. The tested data were then processed with statistic method. Complete C-R-S-N curves were thus fitted and as a result, the values of limits of bending fatigue under various reliabilitys were gained.