Nano sized ferrites, Sr1-xLaxFe12-yCoyO19 with improved magnetic properties have been successfully synthesized by a simple sol-gel method forming Fe2O3 and Co2O3 gel using propylene oxide as a gelation agent, followed by the addition of Sr2+ and La2+ ions. The magnetic behavior of Sr1-xLaxFe12-yCoyO19 powders annealed at and below 900°C showed that an increase of the annealing temperature resulted in the enhancement of the coercivity and the decrease of the saturation magnetization. Especially the highest values of the coercivity and the saturation magnetization of the ferrites with heat treatment were observed at temperature 150°C lower than those of the previously reported values. The highest observed values of the coercivity and the saturation magnetization of Sr0.6La0.4Fe11.8Co0.2O19 were 7,431 Oe and 52.3 emu/g at 900°C. The ferrite powders annealed at 700°C have a crystalline structure of M-ferrite and showed spherical particles shapes. The resulting spheres which were formed by the aggregation of nanoparticles with size 3~5 nm have diameter around 50 nm.