Functionalized carbon black (F-CB) nanoparticles were synthesized by covalently grafting the polyvinyl alcohol on carbon black (CB) surfaces and used as demulsifier to separate the oil from the emulsified oily wastewater. The bottle test showed that the residual oil content in the separated water was as low as ∼ 50 mg/L corresponding to a demulsification efficiency of about 99.90% at an optimal condition within a few minutes. It was believed that the surface wettability of the carbon black could be tuned by modifying with the PVA molecules, which enables the F-CB nanopartilces to be readily migrated to the oil/water interface and have the opportunity to interact with and/or displace the stabilizers of the emulsion. As a result, the demulsification process was accomplished with the coalescence of the oil droplets promoted by the F-CB nanoparticles. The interaction behavior between F-CB nanoparticles and asphaltenes was investigated by quantum chemical calculations. The results showed that the F-CB nanoparticles have strong interaction with the asphaltene molecules in form of π−π and θ−π forces. The findings in present study are significant for understanding the demulsification mechanism and also provide a novel demulsifier for the demulsification of emulsified oily wastewater.