This study investigated the effect of degree of availability (d.o.a) of nitrogen and time on increasing the surface hardness of AISI H13 steel using pack nitriding. Urea (46 % N) and ZA (21% N) were selected as a nitrogen source. In this study, hardening was carried out at a temperature of 1030°C then held for 3 hours and continued to quenching with 19 bar nitrogen. Nitriding process used the in-pack process by which the specimens were buried in the urea and ZA powder charged into a nitriding box. Pack nitriding was performed in a vacuum furnace at 600°C for 2, 4 and 6 hours. The surface of nitride-steels was characterized using microhardness tester, SEM/EDS and XRD. After nitriding process, the hardness of AISI H13 steels was increased up to 1648 HV, with the highest hardness achieved by nitriding process of 6 hours and 0.4 d.o.a, in urea media. The nitrogen was dissolved to the steel and induced precipitation of Fe3N, Fe2N, and αFe. From the calculation based on Arrhenius equation, the activation energy (Q) of 0.4 and 0.3 d.o.a were 1.381 kCal/mol and 1.455 kCal/mol, respectively.