Abstract: The present invention provides a process for producing a coal-water mixture using a reaction mixture obtained by subjecting a low-rank coal of high water content to a hydrothermal treatment in the presence of carbon monoxide and water at a temperature not higher than the critical temperature of water. The above hydrothermal treatment makes the walls of pores present in the coal hydrophobic and densifies the coal matrix, whereby the coal after the treatment has a lower equilibrium moisture content. Hence, it becomes possible to produce a coal-water mixture containing a high concentration of coal and having good stability and fluidity, from a low-rank coal of high water content which has been unusable as a material for a coal-water mixture. Further, in the production of a coal-water mixture from the above reaction mixture, the amount of surfactant used can be reduced because the liquid phase of the reaction mixture contains a large amount of humic acid having surface activity.