PEG-PE (polyethylene glycol-phosphatidyl ethanolamine) of different molecular weight (2000 and 5000) were used to modify the membrane of liposomes. Large unilamellar liposomes containing PEG-PE were prepared by reversed phase evaporation. Fluorescent label-calcein was encapsulated at the internal water phase. To compare the differance between the modified and unmodified membrane, the stability in vitro and distribution in vivo were investigated. The results indicated that the circulation half-life for liposomes unmodified, modified by PEG (2000)-PE and modified by PEG (5000)-PE were 13, 21 and 75 (min) respectively. At 6 h after injection, the ratio b/R (b: distribution in blood, R: distribution in liver and spleen) were 0, 0.8 and 1.4, respectively. The results mean that the stability increased and circulation time was prolonged by the PEG-PE modified membrane. The effect of PEG-PE on membrane was found to be directly proportional to the chain length of the polymer.