Blend films composed of polyaniline and aliphatic copolymer having carboxylic acid groups were prepared by vaporizing a solvent from a solution of both polymers. Poly(methyl methacrylate-co-methacrylic acid) and poly(1-vinylpyrrolidone-co-acrylic acid) were used as the copolymers in this work. Conductivity of the blend film increased with decreasing concentration of polyaniline and then decreased in the region of low concentration. The N-1s peak in the ESCA spectrum of the blend film was shifted to higher binding energy with decreasing concentration of polyaniline and then was done to lower the binding energy in the region of low concentration. Polyaniline chains in the blend films become conductive by being oxidized with the carboxylic acid groups, which are the constituents of the copolymers. Through strong intermolecular interactions such as ionic bonding and hydrogen bonding between polyaniline and the copolymer, densities of the blend films are always higher than those predicted on the basis of the additive law. Blend films with higher density and higher conductivity are able to be prepared with a higher vaporizing rate, given that phase separation occurring during vaporizing process can be depressed.