Small interfering RNA (siRNA), as a potential tool for gene therapy, requires suitable vehicles for stable complexation, protection, low cytotoxicity and high efficiency of gene knockdown. Here, a new polycation, (dextran-hexamethylenediisocyanate)-g-polyethylenimine ((Dex-HMDI)-g-PEI), was used as a siRNA vector to knock down the expression of Enhanced Green Fluorescent Protein (EGFP). It was found that the complexation of siRNA with the polycation can significantly knock down the expression of EGFP, but has no remarkable effect on the expression of luciferase.