PURPOSE: To obtain magnetic particles having preferable acicularity by adding a specific amount of water soluble silicate in the alkaline slurry of iron oxide particles, aqueously heat treating the mixture and subsequently treating it through water rinsing, filtering and drying steps, thereby uniformly modifying the surfaces of the particles with Si.

CONSTITUTION: Aqueous iron oxide particles such as goethite, lepidocrosite or the like are formed in a slurry of pH of 8W14, water soluble silicide is added thereto, the mixture is sufficiently agitated, is subsequently aqueously heat treated in a sealed container such as an autoclave or the like, is thereafter treated through water rinsing, filtering and drying steps, and there can be thus obtained aqueous iron oxide containing SiO2 or iron oxide. The quantity of the added water soluble silicide is 0.1W10mol% per Fe in an SiO2 conversion, and the aqueous heat treatment is conducted at 100W250°C for longer than 5min. This serves to modify uniformly the surfaces of the particles with the Si and also to prevent the growth of the particles, thereby obtaining magnetic particles having preferable acicularity with less branches and improving the recording characteristics of a tape or the like.