To immobilize lead compounds contained in fly ash discharged from waste treatment processes, fly ash was mixed with asphalt, sulfur and calcium hydroxide in a bi-axial kneader. Based on experimental results on sulfuration of PbCl2, PbO and Pb3O4 With sulfur and calcium hydroxide, the condition of immobilization of lead compounds included in five different fly ashes was made clarified. Sulfuration was achieved by heating all lead compounds (PbCl2, PbO, Pb3O4) mixed with sulfur and calcium hydroxide at 523 K. It was found that the lead oxides, PbO and Pb3O4, were sulfurated by the reaction with sulfur only, while lead chloride was sulfurated by calcium sulfide, which was formed by the reaction between sulfur and calcium hydroxide. From the results of a leachability test of stabilized fly ash, it was found that the five fly ashes employed in the present study could satisfy Japan's lead emission standard of 0.3 mg/l when treated under the following conditions: asphalt, 2.5 g/10 g fly ash; sulfur, 0.4-1.2 g/10 g fly ash: calcium hydroxide, 0.2-0.8 g/10 g fly ash.