Crystalline polyethyleneimine (PEI(C)) was successfully grafted onto carbon black surface by direct condensation of imino groups of PEI with carboxyl groups on the surface using N,N’-dicyclohexylcarbodiimide as a condensing agent. The electric resistance of a composite prepared from PEI(C) and PEI(C)-grafted carbon black drastically increased to 10(3)-10(4) times of initial resistance in methanol, ethanol, and water vapor. But the change of electric resistance of the composite hardly observed in hexane and toluene vapor. These results suggest the possibility of detection of a slight change of crystalline structure of PEI by the absorption of solvent as a large increase of electric resistance of the composite.