[en] Graphene is known effective as a short-term solution to protecting metals from oxidation. Past studies have shown that more layers and fewer defects (e.g. grain boundaries) enhance the antioxidation ability of graphene. However, it has remained unanswered which of these two parameters, namely, number of graphene layers and crystallinity of graphene, is more important when it comes to protecting metals. Herein, we aim to study this subject by comparing the oxidation behaviors of Cu covered by two different types of graphene. One is multilayered but contains a high density of grain boundaries, while the other is free of grain boundaries but comes with only one layer. Through oxidizing experiments in air and water-rich vapor conditions, failure mechanisms of graphene and change of underlying Cu (and Cu oxides) are thoroughly investigated using a wide range of analytical tools, including scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). It is found that the single-crystalline, monolayer graphene is more effective than the counterpart in polycrystalline, multilayer format for protecting Cu against oxidation in both air and water-rich vapor conditions. Especially in air oxidizing case, the former graphene raises the temperature barrier by at least 100 °C as compared to the latter graphene. Our results therefore suggest that the crystallinity of graphene deserves more considerations when using graphene as the antioxidizing barrier. (paper)