Abstract: PEGylated cationic liposomes containing 0%-5% (mole fraction) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxy(polyethylene glycol)-2000 (PEG) were studied as siRNA delivery system.Two groups of cationic liposomes containing 1,2-dioleoyl-3-trimethylammoniumpropane (DOTAP) with mole fraction of 20% and 40% respectively were prepared.The kinetic changes on tumor cellular membrane binding and internalization vs different time and different extracellular liposomal concentration were compared.Binding reached the equilibrium at 4 h for all tested liposomes.Cationic liposomes containing 40% (mole fraction) of DOTAP showed higher cell-association amount than those containing 20% (mole fraction) of DOTAP.Cell-membrane binding time course was changed from nonsaturable to saturable kinetics after PEGylation.Cationic liposomes containing 40% (mole fraction) of DOTAP and 0%-2% (mole fraction) of PEG were shown to successfully transfer siRNA to cytosol under confocal microscope, while those with 5% (mole fraction) PEG failed.