The impact wear behaviour of deep plasma nitrided layer and its substrate (axle steel) were investigated. The nitrided layer had a 6 mu m thick compound layer, and the effective case depth was 0.85 mm. Moreover, it introduced compressive residual stress with a maximum of 715.8 MPa at a depth of 511 mu m. The nitrided specimens showed a lower energy absorption ratio and better impact resistance than the untreated specimens. Plastic deformation occurred in the initial test of the untreated specimens, but elastic deformation was predominant for the plasma-nitrided layer. The main wear mechanisms of the untreated samples were pitting, delamination and oxidation wear. In contrast, the plasma-nitrided specimens exhibited better oxidation resistance and slighter peeling-off than the untreated specimens.