A method for manufacturing a metal nanoparticle is provided. The method includes steps of: a) providing a metal salt solution, b) providing a reducing agent, c) providing a protecting agent, d) providing an alkaline solution, e) mixing the salt solution, the reducing agent, the protecting agent and the alkaline solution to form a slurry within a high-gravity field, and f) separating the metal nanoparticle from the slurry.