<p>The oxidation behaviors of a new martensite steel G115 were investigated in ultra-supercritical water in this study. After a short-term oxidation (one hour), a double-layer oxide film, including a porous outer layer of Fe₃O₄ and a thick internal layer, was formed in the surface. The oxidation resistance of Co delayed the complete oxidation of the internal oxide layer, resulting in a thick internal oxide layer. Spinel particles within internal layer exhibited a Baker–Nutting relationship with matrix. Over time, high-angle grain boundaries and Co-rich residual matrix promoted dense Cr-rich oxide layer formation. This Cr-rich layer hindered the inward diffusion of oxygen, causing internal layer to transform into inner layer followed by a renewed internal layer formation. Additionally, the Cr-rich layer hindered the outward diffusion of Fe, thus accelerating the transition of Fe₃O₄ into Fe₂O₃. Cu initially precipitated at the oxide/matrix interface within internal layer, subsequently diffusing beneath the initial Cr-rich layer.</p>