Nano-sized metal particles have been actively investigated. Although the synthesis methods of nano-sized metal particles have been made remarkable progress through many researches, in many cases, heat treatment is required. In order to promote industrial utilization, non-heating, simple and inexpensive synthesis methods are required. In the present study, nano-sized metal particles composed of Ag, Au, Ir, or Pd were prepared in an aqueous solution with a simple and non-heating way. After a noble-metal carboxylate was completely dissolved in the quaternary ammonium carboxylate aqueous solution, nanoparticles composed of the noble metal appeared in the aqueous solution. Since this operation is extremely simple as well as non-heating because the noble-metal carboxylate only dissolves into the quaternary ammonium carboxylate aqueous solution. The nanosized metal particles were observed by Scanning Electron Microscope (SEM) or Scanning Transmission Electron Microscope (STEM) with the freeze-fracture replica method and characterized with Energy Dispersive X-ray Spectroscopy (EDX) and Electron Energy Loss Spectroscopy (EELS).