Abstract: Characteristics of hysteresis loops of stress-strain (total) and stress-strain (plastic) under low cycle fatigue action for 10 Cr Ni Mo structural steel were studied by taking smooth rounded cross-section specimens, in air and at room temperature, under the control of six series of different axial strain amplitudes ranging from 0. 3% to 1. 0%. The results manifested that although elastic strain is entirely dominant, plastic strain is not zero and increases gradually, under the control of the small strain amplitudes from 0. 3% to 0. 4%; plastic strain increases fast and plays a leading role gradually, under the control of the big strain amplitudes from 0. 6% to 1. 0%; the test material will get into a stable state acted only by several cycles despite of the strain amplitudes, and the first cycle and the last one indicate unsteady features.