In this thesis the effect of substituting three rare earth ions in cobalt ferrite lattice on magneto optical properties of prepared thin films were studied. At first effect of initial parameters of sol-gel method on magnetic and structural properties of thin films were studied and optimization process on these parameters were conducted. Then four groups of the series of powdered rare earth ferrite Co x RE (1?x) Fe 2 O 4 , x = 0 - 0.2 in steps of 0.05 and R is Nd, Eu and Dy were prepared using the sol-gel method at annealing temperatures 550 ? C. The materials were characterized by powder X-ray diffraction (XRD) and Field Emission Scanning Electron Microscopes. The phase identification of the materials by XRD reveals the single-phase nature of the materials. The crystallite sizes of the materials were varied by altering the substitution level within the range of a minimum of 16 nm to 22 nm. The magnetic parameters have been studied by using vibrating sample magnetometer. Substituted rare-earth ions inhibit the grain growth of the materials in a systematic manner compared with that of the pure cobalt ferrite materials. There is an improvement in coercivities of the rare earth doped cobalt ferrite especially for 0.15% Neodymium doped cobalt ferrite. Afterward a series of rare-earth (RE)-doped nanocrystalline Co x RE (1?x) Fe 2 O 4 (x = 0 - 0.2 and RE: Nd ,Eu and Dy) thin films were prepared on silicon substrates by a sol-gel process, and the influences of the different RE 3+ ions on the microstructure, magnetism and polar magneto-optical Kerr effects of the deposited films were examined. Also this research presents the optimization process of cobalt ferrite thin films deposited via spin coating, by studying their structural and morphological properties at different thicknesses (200, 350 nm) and various heat treatment temperatures 300 ?C to 850 ?C. Nanoparticulate polycrystalline thin film were formed with heat treatment above 400 ?C but proper magnetic properties due to well crystallization of the film were achieved at about 650 ?C. AFM results indicate that the deposited thin films are crack-free exhibiting a dense nanogranular structure. The root-mean square roughness of the thin films is in the range of 0.2 to 3.2 nm. The results revealed that both of the magnetism and Kerr spectra of Co x RE (1?x) Fe 2 O 4 films could be mediated by doping with various RE ions. In MOKE spectra both dominant peaks were blue shifted with addition of RE ions. For low concentration dopant the IVCT related rotation was enhanced and for higher concentration dopant the ISCT rotation peak was enhanced. The enhancement for Eu 3+ was more than Dy 3+ and Nd 3+ doped cobalt ferrite films. The enhanced MOKEs in nanocrystalline thin films might promise their applications for magneto-optical sensors in adopted wavelengths.