A novel method to prepare nanoparticles using reverse micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer surfactant was proposed. The method is based on the fact that the critical micellization concentration of some PEO-PPO-PEO block copolymers increases sharply with increasing temperature. Therefore, nanoparticles can be synthesized in the reverse micelles at lower temperatures, and the nanoparticles can be precipitated and recovered with a temperature rise because of the breakage of the reverse micelles. As an example, silver nanoparticles were synthesized in the reverse micelles of (EO)(8)(PO)(50)(EO)(8)/p-xylene/H2O and recovered.