Magnetic manganese ferrite nanoparticles were prepared by thermal decomposition. The influence of reaction temperature, reaction time and feeding ratio of oleic acid to oleates on the structure and the property of magnetic nanoparticles were investigated. As reaction temperature and time increasing, the crystal structure of the obtained nanoparticles changed from α-Fe2O3 to manganese ferrite with spinel structure, and then to solid solution of manganese oxide and iron oxide, which resulted in the following increase and decrease of magnetic property. The morphology of the nanoparticles transformed with an intra-particle ripening mechanism. As prolonging reaction time, the shape of the nanoparticles changed from spindle to irregular shape, and then to sphere. When the molar feeding ratio of oleic acid to oleates was 0.8, after thermal decomposition at 300 ℃ for 1 h, the saturation magnetic intensity of the obtained nanoparticles reached to 15.14emu/g.