Abstract: Variation of elastic strain and plastic strain of 10 CrNiMo structural steel in the course of low cycle fatigue action were studied under six series of different axial strain range from 0.6% to 2.0% by taking smooth rounded cross-section specimens. The results indicated that elastic strain range Δε_e increases slightly approximately by the rule of natural logarithm and its ratio in total strain range drops gradually by power rule with a negative exponent; plastic strain range Δε_p aggrandizes linearly and its ratio in total strain range increases by quadratic polynomial rule with the enhancement of total strain range Δε_T; when Δε_T is less than 1.5%, Δε_e is dominant; when Δε_T is equal to 1.5%, proportions of Δε_e and Δε_p are similar and both are dominant; when Δε_T is more than 1.5%, Δε_e increases slowly and Δε_p augments at a high speed and is a dominant factor to control the whole process of low cycle fatigue.