PURPOSE: To obtain an aq. slurry of coal powder with excellent dispersion stability forming no precipitate which becomes hard cake, by mixing pulverized coal with fermentation liquor obtd. by multiplying a petroleum-utilizing microorganism under specified conditions or its supernatant liquid.

CONSTITUTION: A petroleum-utilizing microorganism is multiplied at pH of 6W8 by using a culture medium contg. only hydrocarbon as a carbon source, and the obtd. fermentation liquor or its supernatant liquid is mixed with pulverized coal and, if necessary, with an adequate amt. of water to obtain an aq. slurry of pulverized coal. The petroleum-utilizing microorganism to be used is a microorganism selected from genera Pseudomonas, Corynebacterium, Nocardia, Arthrobacter and Alkaligenes. Said aq. slurry generally consists of 50W75% pulverized coal, 25W40% water and 0.01W10% fermentation product. Coal having such a particle size distribution that 50W90% thereof passes through a 200- mesh screen is used as said coal.