The mechanism of diffusion layer growth in plasma nitrided coatings applied on a St52 steel using an active screen is investigated. The nitriding was performed at 450, 500 and 550 •C temperature nitriding times of 5, 10 and 15 h, in a gas mixture containing 20 vol. % H2: 80 vol. % N2 and DC-pulsed plasma nitriding unit. The surface, cross section and the thickness of diffusion of specimens was studied in terms of optical and scanning electron microscopy. According to the measurements of diffusion layer thickness, values of Q and Do for nitrogen diffusion in substrate were calculated as 50585 (J/mol) and 4.11×10-10 (m 2 /s), respectively. The variations of hardness depth during nitriding period were determined.