This study was conducted to evaluate the effect of ultrasound widely used for extraction of contaminants and for synthesis of complexes, on extraction and stabilization of heavy metals in MSWI fly ash. Ten grams of fly ash was treated at 10 of liquid--solid (L/S) ratio with and without ultrasonic irradiation of 20 kHz. The extractability of Pb from fly ash was enhanced by ultrasound at the acidic condition. At neutral and alkaline conditions, however, ultrasound did not affect the enhancement of extractability. The results of the maximum availability leaching test (MALT) of the residues treated under various pH conditions with or without ultrasound indicated that the stability of Pb in the fly ash residues was enhanced after ultrasonic irradiation irrespective of treated pH conditions. However, there was a critical point on the enhancement of lead stability in the residues even though the extraction efficiency of Pb decreased gradually with increasing ultrasonic intensity during fly ash treatment. Ultrasonic irradiation was effective for phosphate stabilization of Pb in fly ash. Irrespective of leaching tests, less amount of Pb was leached out from the phosphate stabilized residues treated with ultrasound than without ultrasound. The extraction efficiency of Pb from fly ash decreased with increasing treatment time but the fractions available for leaching at MALT increased with increasing treatment time. Conclusively, although the effect of ultrasound on the extraction of Pb from fly ash highly depends on the pH condition of solvent, the stability of lead in the fly ash residues can be enhanced with ultrasonic irradiation.