Stainless steel type 304L has been nitrided in the low pressure (600Pa) and high nitrogen (80% $N_2$+20% $H_2$) environment for 5 hours by the square-wave-pulsed-d.c. plasma as a function of temperature $400{sim}550^{circ}C$ and pulsation. At the lower temperature range of $400{sim}500^{circ}C$ and at the relatively high ratio of pulse duration to pulse period. "S-phase" has been developed in the form of thin nitrided surface layer which has many cracks, leading to be nearly impossible for the industrial anti-wear and anti-corrosion applications. At the higher temperature up to $550^{circ}C$ with the increasing ratio of the pulse duration to pulse period up to $50{mu}s/100{mu}s$, the nitrided layer, whose growth rate has increased also, has been composed mainly of CrN and $Fe_4N$ phases and has become thick, uniform and nearly crack-free.