Carrier doping effects of water vapor and an adsorbed water layer on single-crystal graphene were evaluated. After annealing at 300 °C in nitrogen ambient, the sheet resistance of epitaxial graphene on a SiC substrate had a minimum value of 800 Ω/sq and the carrier density was estimated to be 1.2 × 10 13 cm-2 for an n-type dopant. The adsorbed water layer, which acted as a p-type dopant with a carrier density of-7.4 × 10 12 cm-2 , was formed by deionized (DI) water treatment. The sheet resistances of graphene samples increased with humidity, owing to the counter doping effect. The estimated p-type doping amounts of saturated water vapor were-2.5 × 10 12 cm-2 for DI-water-treated graphene and-3.5 × 10 12 cm-2 for annealed graphene.