Ni-free high nitrogen stainless steels have many applications especially in biomaterials due to properties such as good mechanical properties, excellent corrosion resistance particularly resistance to pitting corrosion and no allergical effects. Pitting corrosion resistance is estimated by PREN value. So that by increasing the PREN value, pitting corrosion resistance is enhanced. For this reason, the purpose of this research is to produce nanostructured high nitrogen austenitic stainless steel (Fe-Cr-Mn-Mo) by mechanical alloying (MA), and investigate the influential factors on improvement of PREN value. For this purpose firstly, the effect of Mo on ? to ? transformation was investigated. Then the influence of particle size of iron powder on the rate of ? to ? transformation was evaluated. Finally, chemical composition was changed for increasing the PREN value. For this purpose, the amount of Mn was reduced and Mo content was increased. MA was performed in nitrogen atmosphere using a high-energy planetary ball mill. Microstructural and phase analysis of the milled powders were carried out employing SEM and XRD, respectively. The absorbed nitrogen contents were determined using nitrogen analyser equipment. In order to investigate the effect of Mo, Fe-18Cr-11Mn-4Mo and Fe-18Cr-11Mn-6Mo powder mixtures prepared from iron powder with mean particle size = 47.34 µm, were milled under nitrogen atmosphere for 100 hr. It was observed that by increasing Mo content from 4 to 6 wt%, the ?/d value was enhanced leading to an enhancement of the phase transformation rate. However, increasing the Mo content led to the reduction of chemical potential for austenite stability. Consequently, completion of phase transformation required more energy and longer milling times. In addition, by decreasing particle size of initial iron powder not only absorbed nitrogen content increased (more effective element for improving the PREN value), but also the rate of phase transformation significantly enhanced. As a result, the necessary milling time for completion of ? to ? phase transformation was decreased from 100 to 8 hr. On the other hand, increasing the rate of phase transformation prepared the possibility of reduction in Mn content from 11 to 8 wt% and also increase in Mo content from 4 to 8 wt%. The results obtained showed that the PREN value could be increased from 35.36 to 55.6 due to the possibility of change in chemical composition. Finally, the results of the heat treatment of the produced high nitrogen steel powder at 1100°C after 1 hr indicated that it has a high thermal stability because of the existence of nitrogen in grainboundaries. So, the variations of grain size were slight after annealing (from 8 to 30 nm). As a result, grain structure remained nanometric. Furthermore, investigation of the amount of nitrogen showed that the amount of nitrogen before and after annealing was nearly constant.