Telah dipelajari pengaruh doping ion alumunium pada nanopartikel cobalt ferrite hasil fabrikasi metode kopresipitasi. Ion alumunium dipilih mengingat jari-jari kovalennya sebanding dengan jari-jari cobalt. Hasil analisis FTIR menunjukkan pada tetrahedral site force constant berubah sebesar 0,23 N/m akibat doping ion alumunium. Berdasar analisis XRD pada puncak tertinggi, ukuran kristalit tanpa dan dengan doping alumuniuma adalah masing masing 57,75 nm menjadi 46,2 nm. Perubahan ukuran kristalit ini disinyalir akibat substitusi ion cobalt dengan ion alumunium dengan jari-jari kovalen alumunium lebih kecil dibanding ion cobalt.It has been studied the aluminum doping effect on co-precipitated cobalt ferrite nanoparticles. Aluminum ion was chosen considering covalent radius comparable to the radius of cobalt. The analysis of FTIR results showed that the change of constant force without and with doping aluminum ion on tetrahedral site was 0.23 N / m. The calculated of crystallite size from the strongest peak of XRD pattern indicated that the crystallite size reduce from 57.75 to 46.20 nm after doped aluminum. The change of the crystallite size was presumably due to the substitution of cobalt ions with aluminum ions since the aluminum covalent radius was smaller than cobalt ions.Keyword : cobalt ferit, aluminum ion, co-presipitation.