Ferro-cobalt magnetic powder in nano-meter size is manufactured to implement in a magnetic fluid sealing mechanism. The powder is coated with silicon dioxide for the prevention of aggregation. Also three surface active agents are applied to improve particle dispersion in fluid medium. The average size of the powder particle is 9.4 nm, and its magnetization is 154 emu/g in the magnetic field of 10 kOe. The dispersed particle shows little settlement for 20 weeks. From the sealing performance test, the holding pressure is 25 times as high as that of a conventional magnetite sealing.