PURPOSE: To prepare a conc. coal/water slurry having high fluidity, long-term stability and uniform quality, by placing specified crushed coal, water, a grinding aid, a dispersant and a pH regulator in a wet mill for grinding.

CONSTITUTION: A slurry having a pH of 8 or higher prepd. by mixing (A) 55W 80wt% crushed coal with an ash content of 7% or higher (B) Na salt of formalin condensate of naphthalenesulfonic acid as grinding aid, (C) hydroxy-polycarboxylic acid (salt) as dispersant which has at least one hydroxyl group and at least two carboxyl groups in one molecule (e.g. sodium malate) and (D) pH regulator such as NaOH, is put in a wet mill for grinding until 60W70wt% coal particles pass through a 200-mesh sieve. The naphthalenesulfonic acid for (B) may have an 18C or lower alkyl group and the condensate has a degree of condensation of 3 or higher. (B) and (C) are used in 0.05W0.2% in total and in a ratio of 9.5:0.5W7:3.