Conventional methods for synthesizing polymer nanoparticles have required large amounts of surfactants, thus increasing the environmental pollution. It is important to develop the synthesis method of nano polymer particles without surfactants. In this study, we proposed a thermally reversible gel as a polymerization field for synthesizing polymer nanoparticles with an average diameter of less than 50nm. In the experiments, methylcellulose (MC) gel, an environmentally and biocoinpatible hydrophilic polymer, was used as a field of soap-free emulsion polymerization of styrene. The mesh width of the gel decreased as the concentration of MC increased. Brownian motion and the coagulation growth of the particles were suppressed due to the three-dimensional network structure of the MC gel. Therefore, the size of the polystyrene nanoparticles decreased. The average size of nanoparticle was 31 nm at MC concentration of 1.0 wt%. In addition, enzymatic decomposition of MC on the particles synthesized in the MC gel improved their dispersion stabilities.