Covalent attachment of low-molecular weight, branched poly(ethylene imines) (PEIs) is able to improve transactiavtor of transcription (Tat) peptide mediated gene delivery. This PEI (600 Da, PEI600) and Tat peptide hybrid has been prepared using a versatile methodology to couple synthetic polymers directly to resin-supported peptides. The hybrid biomaterial combines the unique features of the two original cationic materials and functions as a novel gene delivery vector with good biocompatibility. Various nonviral gene delivery system based upon chemical/plasmid DNA complexes have also been developed to avoid potential problems inherent in viral-gene delivery vectors. Their attractive features include the capability of dealing with large genes or several different therapeutic genes simultaneously, ease of preparation in bulk quantities with high reproducibility and low cost, and ease of administration to patient. This PEI600-Tat peptide conjugates, is expected to have promising applications in gene therapy.