AbstractHigh and very high cycle fatigue regimes are rapidly reached using an ultrasonic fatigue machine working at 20kHz. For instance, a fatigue test up to 109 cycles lasts 14hours at 20kHz while it lasts 100 days at 100Hz. The heating of specimen can reach several tens of degrees at 20kHz fatigue tests. In this work, the S-N curve and the thermal response under fatigue tests at 20kHz were investigated on normalized C45 steel. Fatigue tests at 20kHz demonstrate than fatigue failure exists for number of cycles ranging from 106 to 1010 cycles. In addition, three stress amplitude domains were identified with regard to the specimen thermal response. These domains were correlated with failure response. For , the heating was <10°C and no fracture was observed at least up to 1010 cycles. For , the heating reached few hundreds of degrees. Fatigue fracture took place. The crack initiated at an internal non-metallic inclusion leading to the classical fish eye observed in very high cycle fatigue. For , the heating reached 800°C and a ductile failure occurred.