The influence of plastic strain amplitude and gaseous environment on the low cycle fatigue properties has been studied in a Nb-bearing HSLA steel. The cyclic response under plastic-strain controlled loading is determined. Neither the cyclic stress-strain curve (CSSC) shows a plateau nor persistent slip bunds (PSB`s) have been observed in the bulk. The cyclic plastic strain energy (ΔW) and the accumulated plastic strain energy (W) have been calculated as a function of plastic strain amplitude (Δ_(εΡ)/2) in the form of Κ(Δ_(εp)/2)＾N. The computed values of ΔW and W are in good agreement with the experimental data. The vacuum environment enhances fatigue crack initiation life, fatigue life and W. The environmental effect becomes more severe as the plastic strain amplitude decreases. Tensile properties, cyclic stress response and ΔW are not affected by the gaseous environment.