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We report a vertically stacked graphene tunnel junction with an atomically thin insulating layer for novel function devices. The insulating water layer sandwiched between graphene samples as a tunnel barrier which is fabricated through deionized (DI) water treatment of epitaxial graphene. Two graphene samples fabricated by SiC thermal decomposition are directly bonded to each other in a face-to-face manner. Vertically stacked graphene samples without DI water treated formed an ohmic junction. By inserting the structured water layer as tunnel barrier, the stacked junction exhibits Direct tunneling characteristics in a low-electric-field regime and Fowler–Nordheim tunneling (FNT) characteristics in a high-electric-field regime. The thickness of the structured water layer is estimated to be 0.28 nm by fitting the FNT formula. The very thin structured water layer is stable as tunnel barrier on epitaxial graphene for diode devices, which will have a widely application in electronic devices. [ABSTRACT FROM AUTHOR]