The poly(1-vinylimidazole) (PVIm) with sever al alkyl groups has been synthesized as a new pH-sensitive DNA carrier. The resulting alkylaed PVIm (PVIm-R) was water -soluble in spite of the hydrophobic alkyl groups and the deprotonation of the imidazole groups at physiological pH. Hemolysis assay showed the PVIm-R enhanced membrane disruptive ability at endosomal pH, owing to the protonation of the imidazole groups with a pK(a) value around 6.0. Agarose gel retardation assay proved that the quaternary alkylated imidazole groups worked as DNA binding groups. The resulting PVIm-R/DNA binary complex showed significant gene expression.