We report the fabrication and electrochemical activity of free-standing reduced graphene oxide (RGO) films as cathode materials for lithium ion batteries. The conducting additive and binder-free RGO electrodes with different oxygen contents were assembled by a simple vacuum filtration process from aqueous RGO colloids prepared with the aid of cationic surfactants. The gravimetric capacity of RGO film cathodes showed clear dependence on the oxygen contents controlled by the thermal reduction process. The capacity increased with the increase of the amount of oxygen functional groups, indicating that the main lithium capturing mechanism of RGO cathodes is Li(+) ion interaction with the surface oxygen functionalities. The hydroxyl groups (C-OH) as well as carbon-oxygen double bonds have been identified as the lithiation-active species. The RGO cathodes achieved excellent rate capability due to the fast surface Faradaic reaction, suggesting that self-supported RGO films are promising cathodes for high power application. The graphene oxide (GO)/RGO composite films showed inferior performance to those of RGO only. The poor electronic conductivity of GO might result in inefficient utilization of redox active oxygen functional groups despite the higher oxygen content and higher theoretical capacity of GO/RGO composite films. Further optimization on the amount of oxygen functional groups for higher capacity and better electronic conductivity would lead to the development of RGO based high energy-high power cathodes.