An alkali injection method was used to promote the formation of reduced iron clusters in zeolite Y. By injecting NaOH solution into the pores of iron ion-exchanged zeolite, the iron ions were exchanged back with the injected sodium ions to be precipitated as hydroxide forms. Reduction of these precipitates in hydrogen gas yielded reduced iron clusters inside the zeolite. Ah preparation procedures should be performed under oxygen-free atmosphere since the re-exchange of the injected sodium ions with Fe3+ ions was much more difficult than that with the Fe3+ ions. Mossbauer spectra and ferromagnetic resonance spectra of the reduced catalysts revealed that the iron clusters were extremely small and uniform in size.