The enhancement effect of preadsorbed water on methane storage on activated carbon was experimentally proven. Adsorption isotherms of methane on dry and wet activated carbon with different water content were measured at 275 - 283 K and pressures up to 11 MPa. A sudden change in the methane uptake mechanism that was indicated by the inflection point of isotherms was observed. The measured amount adsorbed always decreased with the increasing water content of carbon before the inflection point. However, a step rise in the isotherm to as much as 63.3% was observed when the pressure reached the inflexion point locating at 4 - 5 MPa. The significant enhancement of water on the methane-storage capacity of activated carbon is expected to considerably improve the adsorbed natural gas (ANG) technology.