This paper demonstrates a novel and simple wafer direct bonding method using fluorine containing plasma activation for Si to Si room-temperature bonding without wet chemical cleaning as well as no requiring annealing. The bonding energy of the bonded silicon wafers increases significantly owing to adding small amount of carbon tetrafluoride (CF4) into oxygen plasma treatment. Thus, very strong bonding strength (~2.4 J/m2) of Si/Si pairs, close to the bulk-fracture strength of silicon, is achieved at room temperature by means of this fluorine containing plasma activated bonding process. The wafer surfaces and the bonding interfaces are investigated by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), respectively. The fluorine containing oxygen plasma treatment results in fluorinated oxide on the Si surface, which may affect the appropriate water molecules adsorbed on the wafer. Therefore, many covalent bonds may be produced in polymerization reaction and contribute to the strong bonding strength at room temperature.