The AISI H13 tool steel are largely used in the manufacture of dies and moulds when high surface hardness and toughness are necessary. The ion nitriding is a surface treatment widely used to improve wear resistance in AISI H13 steel. It is also used as one of the stages of the duplex treatments , were the nitriding precedes another process like PVD ,CVD or TRD. In this work the AISI H13 steel was plasma nitrided in the states “as-received” ,quenched or quenched-tempered ,to evaluate the influence of the microstructure in the formation of the nitrided layers. The materials were investigated by optical microscopy , Vickers microhardness and X-rays diffraction analysis . It was verified that the samples previously quenched and quenched/tempered presented thicker layers . X-rays analysis showed the presence of Fe3N , Fe4N and Cr2N, in all the studied samples. In relationship to hardness it was not verified significant differences among its and the values ranged from 1040 to 1150 HV.