Thermosensitive poly(N-isopropylacrylamide) hydrogel (NIPA hydrogel) beads have attracted much attention due to their applications in reaction and separation processes. This study focuses on the preparation of millimeter-sized, monodispersed NIPA hydrogel beads, for which a novel circulation polymerization technique is proposed. The method involves the drop-wise addition of a pre-gel aqueous solution into swirling silicone oil through a nozzle, and the subsequent conventional free-radical polymerization of the suspended pre-gel droplets, which drifts with the swirling oil. NIPA hydrogel beads that are 3.0 mm in diameter were successfully prepared with a very narrow distribution under fundamental conditions with no coalescence of the pre-gel droplets. The circulation polymerization technique improves the residence time and can be applied to a polymerization system that requires a long gelation time. The size of the resultant hydrogel beads corresponds to the size of the pre-gel droplets that are delivered from the tip of the nozzle, and can be controlled by adjusting the size of the nozzle. (C) 2010 Elsevier Ltd. All rights reserved.