The effect of lipid with covalently attached poly(ethylene glycol) (PEG-lipid) on the surface properties of liposomal bilayer membranes was examined from standpoints of particle size, surface potential, microviscosity, micropolarity, and permeability. The particle size and the absolute value of the zeta potential of liposomes decreased with increasing mole fraction of PEG-lipid. The permeability of PEG-liposomes was decreased as compared with regular liposomes. The microviscosity and the micropolarity of liposomal bilayer membranes showed a maximum and a minimum value, respectively, at 6 mol % of PEG-lipid. The packing state of liposomal bilayer membranes became rigid and the lipid bilayers were microscopically stabilized by the addition of PEG-lipid, and the optimum amount of PEG-lipid in liposomes was 6 mol %.