We have designed and synthesized a peptide, WLFLLKKK (peptide-1), which has positive charges and a segment partitioned into the electrically neutral lipid membrane interface, and investigated its effect on the stability of the phosphatidylcholine (PC) membrane. Spacing of multilamellar vesicles of palmitoyloleoyl-PC increased greatly with an increase in peptide-1 concentration. The addition of 5 muM peptide-1 through a micropipet near the giant unilamellar vesicle (GUV) of dioleoylphosphatidylcholine induced several kinds of shape changes; for example, a discocyte was changed to two spheres connected by a neck, and small vesicles were budded into the outside of the spherical GUV. These results indicate that the de novo designed peptide-1 can be partitioned into the PC membrane interface and have a large effect on its structure and properties.