Thermosensitive microgel particles were prepared by emulsion polymerization using a polymerizable anionic surfactant, SE-10 N. N-isopropylacrylamide and N, N'-methylenebisacrylamide were used as primary monomer and cross-linker, respectively. The gel particles reversibly swelled and shrank in response to temperature change. The effects of synthesis conditions on the particle diameter and the swelling degree were examined. Polymerizable surfactant did not affect the diameter of gel particles, if it was present at above a certain concentration. The gel diameter was dependent on the concentrations of the initiator and accelerator. The swelling degree of gel particles was strongly affected by the concentration of the cross-linker. From these results, the mechanism of particle formation was discussed.