Quenched and tempered samples made of plain carbon steel C 15 were salt bath nitrided at 580(0)C for 210 minutes and subsequently water quenched. The nitrided samples were half cut to obtain ail untreated surface that was polished before further processing. Both, salt bath nitrocarburized and untreated surfaces, were subjected to oxygen containing gas mixture glow discharge in the same bath. The plasma pulse power supply generated 16.6 kHz pulses with 0.95% duty cycle at a temperature of 480(0)C and pressure of 400 Pa for 60 minutes. The thickness of compound surface layer was measured by calotest method, while optical microscopy and SEM analyses were used to investigate the modified surface zone structure and morphology. The phase composition of obtained surface structure was analyzed by XRD. It was found that the salt bath treatment may produce the single epsilon phase or the single gamma' zone on the plain carbon steel specimens depending on the active species concentration in the salt bath. The oxide layer phase formed depends on the applied surface pretreatment.