A novel pH-responsive polymer vesicle obtained by the aqueous self-assembly of carboxy-terminated hyperbranched polyesters is reported. The synthesis is very simple, just a one-step esterification of the commercially available hydroxy-terminated hyperbranched polyester of Boltorn Hx (x = 20, 30, 40) with succinic anhydride. The vesicle size can be controlled from 200 nm to 10 mu m by simply adjusting the solution pH as well as the degrees of branching (or generation).