PURPOSE: To determine the optimal time of an ultrasonic treatment in subjecting a coal slurry to an ultrasonic treatment in the coal liquefaction.

CONSTITUTION: In this process for liquefying coal by slurrying coal in a solvent, subjecting the slurry to an ultrasonic treatment in the presence of a catalyst, and treating the treated slurry in a high-pressure hydrogen atmosphere, the optimal time of the ultrasonic treatment of the slurry from operation factors such as the amount of the slurry and the coal concentration of the slurry and equipment factors such as the number of ultrasonic generators used, their powers and the frequencies is determined. Thus, the optimal conditions for the ultrasonic treatment of the slurry can be rapidly determined in good precision, and the oil yield in the liquefaction reaction can be always kept maximum.