Oxidation kinetic of a Zr55Cu30Al10Ni5 bulk metallic glass (BMG) and its crystalline counterpart were studied under dry artificial air (20% of O2 and 80% of N2) at 673 K by thermogravimetry analysis (TGA) method. According to TGA profiles, the oxidation kinetic in both amorphous and crystalline states followed a protective parabolic law. However, the oxidation rates for the amorphous alloy were obviously higher than those for the crystalline alloy. Pseudo-grazing incident X-Ray diffraction (GIXRD) has been carried out to identify the oxides nature and their crystalline structure. Tetragonal-ZrO2 dominated the oxide scale formed on both alloys (BMG and crystalline) at T = 673 K; meanwhile, a slight amount of Cu was detected on the oxide surface of studied BMG alloy. The atomic diffusion mechanism was investigated using a two-stage oxidation treatment to study oxide scale growth kinetics. The studied specimens were oxidized firstly under dry artificial air and then under 18O2 isotopic tracer gas for 1.5 hours respectively at 673 K. The evident solute penetration zone and ion diffusion characteristic through the oxide scale were determined by Cs+ secondary ion mass spectrometry (SIMS) depth profile. The results showed the mechanism of the oxide layer formation of both alloys was not only due to Oxygen ions diffusion from oxide surface to interior scale, but also to an outward diffusion of Zirconium ions from substrate to oxide layer and the ZrO2 oxide growth seemed to occur at the oxide/gas interface in our studied case.