We have fabricated spin valves which had the simple structure of Si/SiO2/CoFe2O4 (40 nm)/Co (2 nm)/Cu (2 nm)/Co (4 nm) by rf-magnetron sputtering at room temperature. A hard magnetic and insulating Co-ferrite film was used as a pinning layer. As-deposited films showed a high sheet resistance of 36.3 Ω/square and a magnetoresistance (MR) ratio of 4.5%. The MR property of the spin valves was significantly improved by postannealing in a vacuum. An MR ratio of 10.5% was attained by annealing at 300°C for 4 h. The variation in sheet resistance was approximately 3.9 Ω/square. This remarkable increase in MR ratio could be attributed to the construction of clear Co/Cu/Co/CoFe2O4 interfaces and the improvement of the magnetic property of the Co-ferrite layer by postannealing.