The effects of ultrasonic irradiation on the properties of coal water slurry (CWS), such as apparent viscosity, rheological behavior and static stability, were investigated. The ultrasonic intensity was varied from 40 to 80 W cm(-2) and irradiation time from 2 to 10 min. Ultrasonic irradiation distinctly improves both rheological behavior and static stability of CWS with a slight increase in the apparent viscosity of CWS containing 1% additive. The mechanisms having a remarkable improvement on the properties of CWS were proposed by measuring the changes of coal particle size distribution and dissolved ions in CWS before and after ultrasonic irradiation. The ultrasonic irradiation may provide a new technology for CWS preparation. (C) 2000 Elsevier Science Ltd. All rights reserved.