Linear poly(ethylenimine)-graft-poly(ethylene glycol)s (LPEI-g-PEG) with various degrees of grafting and molecular weights were synthesized by Michael addition reaction of LPEI with methoxy poly(ethylene glycol) acrylate. The graft copolymers display pH-sensitive behaviors, and pH variation of the copolymer solution induced the aggregation of graft copolymers, forming the micelles with different sizes. The dynamic light scattering method was used to study the effects of ionic strength, chain composition and pH on hydrodynamic radius. With solvent evaporation of the micelles solution, various morphologies were formed. (C) 2007 Elsevier Inc. All rights reserved.