Metal oxidation initiates from surface adsorption to subsurface and bulk reaction through continuous interfacial phase transformation from metals to oxides. How the initial interfacial process affects the whole process of metal oxidation remains largely elusive because of the lack of direct observation of the evolving interface. Here, through in situ atomic-scale environmental TEM observations of Cu surface reaction in water vapor, we demonstrate that the interfacial strain between the substrate and growing oxide is coupled into the continuing chemical reaction that determines the reaction kinetics. Atomic imaging of the reaction process in real time reveals that the growing oxides could temporarily possess a disordered CuOx phase to lower its interfacial strain with Cu substrate and can transform to a crystalline Cu2O phase later. This flexibility of the oxide phase results from the strong chemomechanical coupling during the interfacial phase transformation, which enhances the oxide penetration into the metal under water vapor.