The article deals with mechanical properties and chemical composition of nitrided layers which were created by plasma nitriding technology. Experiments are focused on usage of plasma nitriding process for surface treatment of cavities with diameter from 6 to 10 mm. Nitrided layers were applied to steel AISI 4140 which were subsequently evaluated by metallographic, GDOES, XRD microanalysis and microhardness method. The results of experiments showed that plasma nitriding process is applicable to mentioned cavities. Surface hardness of material was significantly increased. The measurements showed that surface hardness is dependent on content of nitrogen in material.