Abstract: In this paper, the ultra-fine particles of Fe₃O₄ coated with a single layer of molecular oleic acid were prepared by the neutralizing co-precipitation solution-adsorption process. The particle size, shape and structure of Fe₃O₄·nOA wore primarily studied by electron microscopy, infrared spectroscopy, DTA-TG, X-ray diffraction, etc. Some of the methods of calculation and measurement of the particle size and quantity of oleic acid in Fe₃O₄·nOA were provided by testing and supposition.