In ITER D-T fuel will be used for fusion plasma. Therefore a material capable of acting as a tritium permeation barrier on stainless steel is required. It is well known that thin alumina layer can reduce the tritium permeation rate by several orders of magnitude. A technology is introduced here to form a ductile Fe/Al layer on the steel with an alumina over-layer, which consists of two main steps, hot-dip aluminizing (HDA) and subsequent oxidation process. According to the experiments that have been done, Fe-Al intermetallic layer and outer aluminum coat were formed after the HDA process. In this report, we have studied the influence of temperature and oxygen pressure on the formation of ductile phases and alumina over-layer in the oxidation treatment. At the temperature of 900°C, the brittle Fe-Al intermetallic layer and aluminum coat transformed into ductile phase (FeAl and α-Fe(Al)) and a γ-Al2O3 over-layer was formed under the oxygen pressure about 5Pa. It was found that the low pressure of environment suppressed the formation of pores or cracks as well.