In order to improve the magnetic and chemical stability of Co-magnetite fine particles, silica and aluminum hydrate coating onto the surface of Co-magnetite particles in aqueous slurry is investigated experimentally. The time-dependent change of coercive force and Iron(II) content for Co-magnetite fine particles are lessened by this silica and aluminum hydrate coating method. This means that the magnetic and chemical stability is improved by this wet-type coating method. It is considered that these Co-magnetite fine particles are protected from oxidation sufficiently enough because of the coating layer of 0.5 similar to 1.0nm in thickness based on silica and aluminum hydrate surface treatment. This experimental result is roughly in agreement with the estimated thickness based on the uniformly covered surface layer model. When coated Co-magnetite particles are applied to magnetic recording media, there is no disadvantage of tape properties by silica and aluminum hydrate coating, and it becomes clear that the available magnetic tape with both good magnetic property and good stability can be prepared in this investigation.