The leaching properties of harmful trace elements were compared in four municipal incineration fly ashes: one from a typical medium-scale stoker furnace in Japan, and three from large-scale fluidized bed furnaces at three representative large cities in China. The Chinese ash samples had larger diameter, higher bulk density and different elemental contents from the Japanese ash. In column experiments with constant water head using 0.1 mol/L nitric acid solution, the Chinese ashes showed higher elution speed, while breakthrough, i.e., the rapid increase in the elution speed with decrease in pH due to elution of major and minor elements, was also observed, though within a shorter period than with the Japanese ash. The concentration changes of Pb, Cd, Cr and Mo were measured until steady elution was observed after breakthrough. The elution behavior of each element was similarly affected by changes of elution speed, especially by the phenomenon of breakthrough, despite large differences among the ash samples in elution rate and in the content and source of elements affecting their valences and other properties. Pb, the most abundant trace element in ashes, showed typical amphoteric metal properties. Leaching of Cd was found from neutral to acidic pH of the effluent. The elution rate of Cd from the Chinese ashes was much lower than that from the Japanese ash, which suggested the existence of mass transfer resistance between particles. Cr was leached out in greater amount than Pb at alkaline pH, but even under strongly acidic conditions its elution rate was low. Mo was eluted under alkaline conditions, but its concentration gradually decreased. The solubility data of hydroxides of elements under neutral and acidic conditions well explained the elution of Cd from neutral condition. They also suggest that the breakthrough phenomena are caused by the dissolution of Al and Fe(3+) ions at the column top, following deposition of their hydroxides by dissolution of alkaline elements, then their re-dissolution by acidification of solution.