Metallicnanoparticles embedded in a polyimide (PI) matrix were fabricated through selectively oxidizing a layer of Ni80Fe20 metal film sandwiched between two PI precursor layers. Ni nanoparticies, formed in a monolayer between two PI layers, had an average particle size of similar to5 nm. X-ray photoelectron spectroscopy confirmed that Fe in the film was preferentially consumed, resulting in formation of Ni nanoparticles. Similar experiments to produce uniform-sized Co nanoparticles failed to produce uniform-sized Co nanoparticles when a Co80Fe20 film was inserted because, unlike Ni, Co was partially oxidized during imidization. Transmission infrared spectroscopy suggested that thermal degradation of the PI matrix was catalyzed by the Co80Fe20 film, leading to the Co oxidation together with Fe.