The low temperature plasma nitriding behaviour of 17-4PH type precipitation hardening stainless steel was investigated in the present work. The morphology, thickness, hardness and X-ray difraction characteristics of the nitrided layers produced at temperatures between 350degreesC and 450degreesC were studied It was found that, at temperatures lower than 425degreesC, a thin, hard nitrided layer exhibiting a 'white' and featureless morphology could be produced on the steel investigated X-ray diffraction analysis revealed that this layer was X-raj; amorphous, as derived from its lack of Bragg reflection peaks in the diffraction pattern. At higher nitriding temperatures, chromium and iron nitrides began to form it? the nitrided layer, leading to a transition in nitriding kinetics and a decrease in hardness. The possible mechanisms relating to the formation of the amorphous-like nitrided layer at sufficiently low temperatures are discussed in terms of solid state fusion reaction for of the theory of solid state dif amorphous alloy formation.