The microencapsulation of islets of Langerhans (islets) has been studied as a safe and simple technique for islet transplantation without the need for immuno-suppressive therapy. However, thinner membranes are desired, because the increased total volume of the implant led to limited transplantation sites. Here, we propose a novel method for microencapsulation by polyion complex membrane formation on islets. Amino group-terminated poly(ethylene glycol)-conjugated phospholipids (PEG-lipids, M(w): 5000) spontaneously formed a thin layer on cells existing in the outer layer of islets when they were added to islet suspension. This layer-by-layer membrane could be further formed on the PEG-lipid layer through polyion complex formation between amino groups at the end of PEG chains, sodium alginate and poly(l-lysine). Islets could be microencapsulated by this method without volume increase. Encapsulation of the islet surface with PEG-lipids and polyion complex membranes did not impair the insulin release function in response to glucose stimulation. Our method is promising to encapsulate islets without affecting cell viability or increasing volume.