The poly(1-vinylimidazole) (PVIm) with aminoethyl groups has been synthesized as a new pH-sensitive DNA carrier. The resulting aminated PVIm (PVIm-NH2) was water-soluble in spite of the deprotonation of the imidazole groups at physiological pH, as determined by acid-base titration and solution turbidity measurement. Hemolysis assay showed the PVIm-NH2 enhanced membrane disruptive ability at endosomal pH, owing to the protonation of the imidazole groups with a pKa value around 6.0. Agarose gel retardation assay proved that the introduced aminoethyl groups worked as an anchor groups to retain DNA. The resulting PVIm-NH2 formed the ternary complex with DNA and lactosylated poly(L-lysine), leading to the significant gene expression in human hepatoma HepG2 cells.