The formation of an ultraflat surface on copper thin films is important for suppressing the oxidation process, but the formation of multi-atomic steps and terraces is a result of the basic atomic stacking process and difficult to avoid. Here, we develop an innovative strategy for fabricating copper thin films with a semi-permanently oxidation-resistant surface. Comprehensive atomic-resolution microscopy reveals that our thin film consists mostly of flat surfaces and occasional mono-atomic steps. Our first principles calculations show that a mono-atomic step edge is as impervious as a flat surface to oxygen consistent with our experimental observations. Furthermore, the first principles calculations exhibit self-regulated oxidation resistance at the top surface layer: when the oxygen coverage of fcc sites of the surface exceeds 50%, the adsorbed oxygens on the top layer suppresses further adsorption of additional O atoms and renders the ultraflat Cu surface to be more oxidation resistant.