Polyethylenimine (PEI) is one of the most efficient and widely studied synthetic carriers,which have been proven to be an effective transfection agent for the delivery of plasmid DNA into cells both in vitro and in vivo.Low molecular weight PEI has low cytotoxicity but very poor transfection activity [1].Here,we used biodegradable block copolymers to modify low molecular weight PEI in order to enhance its transfection efficiency.