Magnetite ultrafine particles (-10nm in diameter) were synthesized from an aqueous solution of FeCl₂+FeCl₃, utilizing Fe²⁺→Fe³⁺ oxidation by air (oxygen). Their dispersibility in water was measured by absorbance of light recorded as a function of time. The magnetite ultrafine particles was found to be dispersed stably in aqueous solution when pH≥5 and pH≤11. The surfaces are charged positively and negatively at these acidic and alkaline conditions, respectively. This allows the magnetite ultrafine particles dispersed stably in water due to repulsive force of the surface charge. The bit pattern of a floppy disk was successfully observed using the magnetite ultrafine particles dispersed in water at pH=5.