We have successfully prepared monodispersed thermoresponsive core-shell hydrogel microspheres with a mean diameter of 200-400 nm with poly(N-isopropylacrylamide-co-styrene) [P(NIPAM-co-St)] cores and poly(N-isopropylacrylamide) (PNIPAM) shells. The submicrometer-sized monodispersed P(NIPAM-co-St) core seeds were prepared by using a surfactant-free emulsion polymerization method, and the PNIPAM shell layers were fabricated onto the core seeds by using a seed polymerization method. The particle size, morphology and monodispersity, and thermoresponsive characteristics of the prepared microspheres were experimentally studied. In the preparation of P(NIPAM-co-St) seeds, with increasing the initiator dosage, the mean diameters and the dispersal coefficients were almost at the same levels at first; however, when the initiator dosage increased further to a critical amount, the mean diameters decreased drastically and the monodispersity became worse significantly. With increasing the stirring rate, the particle diameter decreased, and when the stirring rate was larger than 600 rpm, the monodispersity became worse obviously. With increasing the phase ratio, the mean diameter became larger simply, and the monodispersity became worse first and then became better again. With increasing the reaction time, the particle sizes nearly did not change, while the monodispersity gradually became better slightly. For the core-shell microspheres, with increasing the NIPAM dosage in the preparation of the PNIPAM shell layers, the mean diameters became larger simply, the monodispersity became better, and the thermoresponsive swelling ratio of the hydrodynamic diameters increased.