Abstract: Tensile specimen of X80 pipeline steel was strained to over 1% plastic deformation in air. The flow stress of a X80 linepipe steel extended over yield is larger than the yield stress of the same sample extended in" air after unloading and charging with hydrogen. The difference is the hydrogen-induced additive stress, which can help the external stress to enhance the plastic deformation. The hydrogen-induced additive stress, σ_ad, increases linearly with the increase of the hydrogen concentration, i. e.σ_ad=- 14. 1 + 3.89 C₀.