Low temperature plasma assisted nit riding treatments of 316 stainless steel produce a complex layer constituted by tow different metastable f.c.c. solid solution denoted ( γ _N1 and γ _N2 ). About the formation of these double layers, different hypothesis was proposed in the literature. For verifying these hypotheses, the effects of differentes conditions such as nit riding temperature, cleaning and nit riding duration and cooling state have been studied. The results show that γ _N2 sub layer produce during the ion bombardment cleaning procedure, before the nit riding treatment. Also the formation of the γ _N2 layer is not connected to the cooling state of the sample after nit riding treatment.