H13 steel was treated by arc Plasma Assisted Nitriding with different Nitrogen-hydrogen flow (sccm) ratios of 50:25, 38:38 and 25:50 respectively.The structure phases,deepness,surface morphologies ,cross-sectional element distributions of the nitrided layer were investigated by scanning electron microscope(SEM),optical microscope ( OM ) ,X-ray diffraction(XRD) and electro-prob microanalyzer(EPMA).The results show that the nitrided layer is composed of compound layer (ε-Fe3N )and nitrogen diffusion zone(α-Fe(N)).Compare to a higher Nitrogen-hydrogen flow ratio, at the Nitrogen-hydrogen flow ratio of 25:50,the thickness value of compound layer reach a min mum 0~0.3 μm,but the deepness value of nitrogen diffusion zone reach the maximum of 40~45 μm.Nitrogen diffusion zone show increased deepness value with the increasing of hydrogen partial pressure .And,in a certain rang of flow ratio, cross-sectional element distributions tend to be gentle as the increasing of hydrogen partial pressure.But the highest surface nitrogen content is formed at the nitrided layer which is formed the thickest compound layer ,at the Nitrogen-hydrogen flow ratios of 38:38.The surface morphologies (fig.1) show that the sputter crater and preferential sputtering are influenced by Nitrogen-hydrogen flow ratio.