A phospholipid derivative represented by the following formula (I) wherein R.sup.1CO and R.sup.2CO independently represent an acyl group; R.sup.3 represents hydrogen atom, or a hydrocarbon group; symbol "a" represents an integer of 0 to 4; symbol "b" represents 0 or 1, provided that when a is 0, b is 0; X represents hydrogen atom, an alkali metal atom, an ammonium, or an organic ammonium; A.sup.1O and A.sup.3O represent an oxyalkylene group containing oxyethylene group, wherein the ratio of the oxyethylene group to the oxyalkylene group in A.sup.1O and A.sup.3O is 0.5 or larger in terms of a weight ratio; A.sup.2O represents an oxyalkylene group; symbols "m" and "q" represent an average molar number of added oxyalkylene groups; and symbol "n" represent an average molar number of added oxyalkylene groups; provided that m, n and q satisfy the following conditions: 5.ltoreq.m.ltoreq.600, 1.ltoreq.n.ltoreq.45, 0.ltoreq.q.ltoreq.200, 10.ltoreq.m+n+q.ltoreq.600, 0.04.ltoreq.n/(m+n+q), and q/(m+n+q).ltoreq.0.8, which can thicken the water shell of liposome surface by suppressing the spreading of the polyalkylene oxide structure on the surface and thus increase stability of the liposome ##STR00001##
