Novel multiwalled carbon nanotube graft polyethylene glycol-block-polystyrene copolymer nanohybrids (MWCNTs-g-PEG-b-PS) were prepared via atom transfer radical polymerization using carboxylated MWCNTs (MWCNTs-COOH) as a conductive carrier. The chemical structure and the compositions of graft copolymer nanohybrids were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetry, and gas chromatography, and the morphologies and dispersion behavior were observed by ultraviolet visible spectrophotometer, scanning electron microscope, and transmission electron microscope etc. The MWCNTs-g-PEG-b-PS-grafted copolymer nanohybrids were fabricated into conductive composite thin film sensors to investigate their responsiveness to benzene vapor. The influence of the molecular weights or compositional ratios of PEG and PS blocks on the conductivity of the conducting polymer nanohybrids and responsibility towards benzene vapor was investigated. The conducting polymer nanohybrid film sensors were demonstrated to possess the characteristics of quick response and good restorability, repeatability, and stability.