Abstract: Inflences of the pH value of solution, reactants concentration as well as their ratio and reaction temperature on the particle size, distribution and composition were studied. It showed that the pH value in reaction system should lie in the range of 4.4 to 5.0; the particle composition depended only on the initial ratio of reactants; the properties of particles could be controlled effectively by altering the concentration and ratio of the reactants; the reaction temperature had less effect on particle size and distribution. The composite Al_2O_3Cr_2O_3 ultrafine powders synthesized were spherical with homogeneous composition.