Optical and electron microscope investigations showd d that the growth rate of unit particles of ferric oxide at high temperatures is of dependence upon their initial size of the unit particles and their aggregates before heating. In detail : (1) The unit particles with smaller initial size show higher rates of growth at higher temperatures. (2) The size of unit particles heated at constant temperatures is affected by the initial size of their aggregates. The growth of unit particles caused by the material transport between their aggregates is acceralated by heating at higher temperatures. (3) The growth rate of unit particles at higher temperatures is greater for the sample compacted than for the loose sample.