Polyethylene (PE) was grafted onto carbon black surface by gamma-ray radiation of the PE-adsorbed carbon black. Vapor sensor composite materials were prepared from the PE-grafted carbon black and PE as a matrix polymer. The effects of heat-treatment and gamma-ray radiation-treatment on the response of the electric resistance of the sensor material against cyclohexane vapor were investigated. The heat-treatment of the composite improved the crystallinity of the matrix PE, and thus increased the responsiveness against cyclohexane vapor about five times that of the untreated one. The gamma-ray radiation-treatment slightly decreased the responsiveness against cyclohexane vapor, because the gamma-ray radiation-treatment induced the crosslinking of PE. On the contrary, the stability and reproducibility of the vapor sensor material remarkably improved. By the heat-treatment followed by the gamma-ray radiation-treatment, a novel stable and reproducible sensor material was obtained, which allowed to identify and to quantify certain vapors in air accurately.