Small size gold (Au) and rhodium (Rh) nanoparticles have been produced by flowing the solution containing Au(III) and Rh(III) ions with poly(N-vinyl-2-pyrrolidone) through a high-temperature and high-pressure reactor. Au, mostly monometallic, particles (7.5 nm) are produced together with the smaller Rh nanoparticles (2.5 nm) within a few seconds, of which the structure and sizes are identified by the transmission electron micrograph (TEM) images and the extended X-ray absorption fine structure (EXAFS).