Methane chemical vapor deposition (CVD) was conducted to enhance the conductivity of activated carbon (AC) and control its surface functional groups. The effect of methane decomposition on the conductivity and surface functional groups of AC was investigated. Methane decomposition was treated on 5 g AC into a tubular reactor at 900 °C, with reaction times ranging from 5 to 30 min. The conductivity of the AC after methane decomposition increased by 117%, and surface functional groups were controlled up to 85%. As the reaction time increased, the conductivity of AC increased and the surface functional group content decreased. ACs in the section where conductivity significantly increased after the reaction were selected and named according to reaction temperature and time, and changes in characteristics of AC were examined. As a result, the activated carbon surface carbon was converted from sp3 to sp2 structure by methane CVD, and the effects of improved conductivity and reduced surface functional groups were confirmed. In this study, we confirmed that the effects of the methane CVD reaction on improving the conductivity of AC and controlling surface functional groups. Moreover, the mechanism of AC surface treatment by methane decomposition was analyzed and discussed.