This study examined the effects of the mechanical milling conditions on the hydrogen adsorption behaviors of graphite flakes under different gas streams. A ball mill technique with various gas streams during treatments was used to introduce oxygen-containing functional groups on the graphite surfaces. The structural properties of graphite were evaluated by XRD, and the surface properties and textural properties were observed SEM, FT-IR, XPS and N2/77 K adsorption isotherms. The hydrogen adsorption behavior of the graphite flakes were evaluated using a volumetric method at room temperature and 100 atm. The mechanically-milled graphite flakes under an oxygen stream showed a higher concentration of oxygen functional groups and greater hydrogen adsorption capacity than that of graphite flakes under an argon stream. This suggests that oxygen functional groups have good chemical affinity with hydrogen molecules in this system.