In this work, the effect of nitriding on 4140 alloyed steel was investigated by exposing it to a post-discharge nitriding process at 847 K and holding times of 30, 60 and 90 min. The effects of holding time on the microstructural changes were investigated using the scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, corrosion test and hardness measurements. The results confirmed formation of γ′-Fe4N and ε-Fe3N phases in compound layer in all treatment times. The amount of ε-Fe3N phase decreased as the nitriding time increased. Samples nitrided during 90 min provided the highest corrosion resistance among all holding time.