The methane adsorption of water-preadsorbed carbons of different micropore widths w at 303 K was measured. Although the amount of adsorption of supercritical methane on microporous carbon at 303 K was less than 9.4 mg g(-1) at 101 kPa, the presence of the preadsorbed water enhanced noticeably the methane adsorption at 303 K even under subatmospheric pressure. The adsorption increment of methane reached a maximum at 1-2 h after introduction of methane and decreased gradually to a steady value after 20-50 h. The adsorption increment of methane depended on the fractional filling phi(w) of micropores by the preadsorbed water. The maximum increment of 110 mg g(-1) for w = 1.1 nm at a methane pressure of 2.6 kPa was obtained at phi(w) = 0.34, corresponding to the estimated adsorption amount at 21 MPa of methane (130 mg g(-1)). The methane-adsorption increment increased linearly with phi(w) until phi(w) = 0.35, indicating the formation of the stable methane-water clathrate of which the composition of methane to water is 1:2. Thus, the nano-order hydrates of methane should be formed in the micropore. The plausible model of the nanohydrate was proposed on the basis of the experimental results and simulation of methane adsorption.