Vesicles of anionic phospholipid, dimyristoylphosphatidylglycerol (DMPG) were prepared by suspending the lipid film in aqueous solutions of different Na+ concentrations. These vesicles were revealed to show ionic-strength-dependent properties at Na+ concentrations lower than approx. 100 mM. Thus, the mean diameters and multiplicities of the vesicles obtained by negative stain electron microscopy increased linearly with an increase in Na+ concentration, similar to the transition enthalpy of the gel to liquid crystal phases of the vesicles by calorimetry. Furthermore, a marked broadening of the transition peak with a decrease in Na+ concentration proceeded simultaneously with a gradual growth of shoulders at the high temperature side. These results were discussed from screening effects of Na cation interposed between the bilayers of DMPG on negatively charged, concave and convex surfaces of their inner and outer membranes, particularly focusing on the effective cross-sectional area occupied by the charged head group, related to the lipid packing.