A crystalline block copolymer of poly(ethylene-block-ethylene oxide) (PE-b-PEO) was successfully grafted onto a carbon black surface by direct condensation of its terminal hydroxyl groups with carboxyl groups on the surface using N,N'-dicyclohexylcarbodiimide as a condensing agent. The electric resistance of the composite from PE-b-PEO (PEO content is above 50 wt %)-grafted carbon black drastically increased to 10(4)-10(6) times of the initial resistance in a vapor of dichloromethane, chloroform, tetrahydrofuran, and carbon tetrachloride, which are good solvents for PE-b-PEO, and returned immediately to the initial resistance when the composite was transferred in dry air. However, the change of the electric resistance of these composites was less than one-tenth in a poor solvent vapor at the same condition. The response of the electric resistance was reproducible and stable even after exposure to a good solvent vapor and dry air with 30 cycles or exposure to the vapor over 24 h. The effect of PEO content on the vapor response was also investigated. The composite from PE-b-PEO-grafted carbon black responded to the low vapor concentration with a linear relationship between the electric resistance and the concentration of the vapor in dry air. This indicates that the composite can be applied as a novel gas sensor.