Two stainless steels DIN 1.4307 and DIN 1.4306 were tested by an ultrasonic fatigue loading device under push-pull mode (stress ratio R = -1). Life-time curves were determined in the high and very high cycle fatigue regions. The time dependence of frequency was monitored and evaluated. At the beginning of the tests (up to 1 × 107 cycles), an increase of the frequency was observed, followed by a slow frequency decrease. It relates to changes of microstructure. No failure occurred in the region between 1 × 109 and 1 × 1010 cycles. The steel 1.4306 exhibits a higher fatigue life-time than the steel 1.4307. This is caused by the difference in the chemical composition of the two steels, namely by the higher nickel content in the 1.4306 steel.