In the present study, effect of N alloying on microstructural evolution and mechanical properties of metastable austenitic steel with a composition of Fe-4Cr-16Mn-0.1C (in wt%), during martensite reversion thermomechanical treatment was investigated. The main goal of this work was enhancing the mechanical properties of this steel using a proper combination of N alloying and grain refinement by martensite reversion thermomechanical treatment. To investigate the effect of N, four different alloys containing zero, 0.11, 0.21 and 0.25 wt% N were produced. In the martensite reversion thermomechanical treatment stage, all spcimens were cold- rolled up to 40 % thickness reduction; then reversion annealed at 750, 900 and 1000 ?C for 1, 5, 10, 50, 100 and 1000 seconds. Characterizaition of specimens was done by means of XRD, Vickers hardness, and uniaxial tensil tests. Optical and scanning electron microscopes were used to study the microstructural evolutions. Results showed that the volume fraction of phases (austenite, ??-martensite and ?-martensite) changes with increasing of N. It was observed that increasing N increased the volume fraction of austenite but decreased that of ??-martensite. Microstructural investigations after reversion annealing revealed that, in spit of decreasing the volume fraction of strain-induced martensite with N increasing, the final average grain size decreased. In addition, a bimodal grain size distribution was formed in the highly N-bearing alloyes. Tensile test results showed that yield strength enhanced with increasing N in the expense of ductility. It was found that N impact on mechanical properties can be explained in two different ways; first, increasing yield strength due to solid solution strengthening, and second, postponing the onset of strain induced martensite formation to higher strains. In this work the highest yield strength and ultimait tensile strength were 825 and 1230 MPa, respectively, that belongs to the 0.25 wt% N alloy. Key words : austenitic steel, martensite reversion thermomechanical treatment, Fe-Cr-Mn-C-N steel, grain refinement.