The present study is concerned with sulfidation treatment of incineration fly ashes containing heavy metals such as lead, zinc and copper with sodium sulfide in order to convert the heavy metals in the fly ashes to heavy metals sulfides. Two types of fly ashes from municipal waste and industrial waste incineration units were employed. The molar ratio of sulfidation agent (Na2S) to the content of heavy metals in the fly ashes, was varied from 0.15 to 1.5. The sulfidation treatment was conducted by mixing fly ash with Na2S solution in a plastic container, which was then shaken vertically at amplitude of 5 cm and at a speed of 350 spin for 1-48 h. At the end of the experiment, the slurry was filtered using a 1 mu m pore size filter. The concentrations of heavy metals measured in filtrates were used to determine an optimal molar ratio of Na2S to heavy metals. In addition, the conversion of heavy metals to heavy metal sulfides was calculated based on the consumption of sulfide ions in filtrates using a sulfide ions selective electrode. As a result, it was found that the concentrations of heavy metals, Pb, Zn, Cu, in the filtrate were lower than 0.1 mg.L-1, when the fly ash was shaken with Na2S solution for 3 h at a molar ratio of Na2S to heavy metals of 0.9. After 48 h, total conversions of all heavy metals in fly ash to metal sulfides determined at a molar ratio of Na2S to heavy metals of 1.2 were 86.7% and 81.2% for the fly ashes from municipal waste incineration unit and industrial waste incineration unit, respectively.