Abstract: The variation of magnetic field intensity Hp(y) and the absolute value of magnetic gradient |k| with tensile stress of 34CrMo4 steel were studied during tensile testing. It was found both Hp(y) and |k| increased with the increase of σ in the elastic stage, and reached the maximum values closing to yielding stress. However, Hp(y) and |k| decreased abruptly in the plastic stage, which was explained in detail in the paper.