The magneto‐optical Faraday effect of plastic foils composed of cobalt fine particles fixed by polystyrene was investigated. Colloidal suspensions of the cobalt particles from 5 to 10 nm in diameter were prepared by thermal decomposition of dicobalt octacarbonyl dissolved by toluene. After that, polystyrene was added to the solution. The solution was then dried on glass receptacles, and foils 0.1 to 0.2 mm in thickness were obtained by peeling it off from the receptacles. The Faraday rotation spectra &thgr;F(&lgr;) of the foils were quite different from those of a cobalt film and polystyrene. &thgr;F(&lgr;) varied with the particle size and agglomerate state of particles during solidification.