Giant vesicles (GVs) composed of zwitterionic phospholipids were prepared by the electroformation method. The growth behavior of GVs was quantitatively analyzed as a first-order kinetics of the radius of GVs to obtain the apparent growth rate constant k(Gr). On the basis of the dependence of the k(Gr) value on the preparation temperature and the lipid composition of GVs, the membrane fluidity of vesicle membranes was found to be dominant in the growth behavior of GVs. The comparison of the k(Gr) values with the membrane fluidity for GVs suggested the validity of the model that the swelling of the lipid membrane could induce the growth of GVs.