Abstract: PEG-PE(polyethylene glycol-phosphatidyl ethanolamine)of different molecularweight(2000 and 5 000)were used to medify the membrane of liposomes.Large unilamellarliposomes containing PEG-PE were prepared by reversed phase evaporation,Fluorescent label-calceinwas encapsulated at the internal water phase.To compare the differance between the medified and unmedified membrane,the stability in vitro and distribution in vivo were investigated.The results indicated that the circulation half-life forliposomes unmedified, modified by PEG(2000)-PE and modified by PEG(5000)-PE were 13,21and 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 incrcased and circulation time was prolonged by the PEG-PEmedified membrane,The effect of PEG-PE on membrane was found to be directly proportional to thechain length of the polymer.