Surfactant-free thermoresponsive microspheres with a silicone oil cores surrounded by poly(N-isopropylacrylamide) shells have been successfully prepared by physical coating method for the first time. The influences of reaction temperature, N-isopropylacrylamide (NIPAM) dosage, and stirring rate on the formation, morphology, particle size, and monodispersity of microspheres were experimentally studied. In the preparation of microspheres, when reaction temperature was above the lower critical solution temperature of poly(N-isopropylacrylamide), products had higher yield of particles and narrower size distribution. With increasing NIPAM dosage, the particle diameter became larger and the shell layer thickened and the monodispersity became better. With increasing stirring rate, the particle diameter and the monodispersity decreased obviously. (c) 2006 Wiley Periodicals, Inc.