Nanoparticles with different morphology and composition were fabricated inside a polyimide (PI) matrix based on selectively oxidizing a layer of Fe(100-x)Pt(x) alloy metal film sandwiched between two PI precursor layers. Gamma-Fe2O3, Pt, and Fe3Pt nanoparticles were formed in a monolayer between two PI layers, depending on the alloy film composition and curing conditions. These particles were well-crystallized and sized between 4 and 10 nm. X-ray photoelectron spectroscopy confirmed that Fe in the film preferentially reacted with the organic matrix whereas Pt remained metallic throughout the curing process, which enabled fabrication of particles different morphology and composition. This process can be easily extended to other alloy films, which provides an opportunity to fabricate nanoparticles relatively easily with desired composition and morphology embedded in an inert organic matrix.