The invention discloses an ultrasonic assistant hydro-thermal synthesis process for magnetic Fe3O4 nanometer super 
fine grain. The invention solves the problems of impure products and large grain 
diameter. The invention has the technical proposal that the form of Fe3O4 
crystal nucleus is promoted in the 
cavitation effect of ultrasonic; 
cavitation gas bubble is produced in mediums; the pH value of strong 
ammonia is adjusted to 11 to 13; and the nanometer grain 
diameter size and the magnetic characteristics are controlled by controlling the hydro-thermal synthesis temperature within range from 140 DEG C to 160 DEG C and within time of 3 to 5 hours, and by changing the 
micro environment of the 
air bubble chamber, so as to obtain the dry product which is black magnetic Fe3O4 nanometer 
powder after being milled. The invention comprises no 
organic solvent, belonging to full green 
environmental protection which is a break in the field, and the invention is also the first to use the 
ultrasonic technology for assistant hydro-thermal synthesis. The invention has the advantages of simple technology, low cost, small 
powder granularity and even grain 
diameter. The 
powder not only can be used as duplicate ink powder, but also can be board used in aspects such as medical, biological technology, magnetic immunocyte separation, 
DNA separation, 
nucleic acid hybridization, and preparation for magnetic butt directional 
medicine carrying micron ball.