The possibility of hydrothermal treatment (HTT) of brown coal at 200-350 degrees C was examined as a pretreatment method for improving the space time yield of a Victorian brown coal liquefaction reactor. Main reactions occurring during HTT below 350 degrees C were decarboxylation and dehydration, by which most of carboxylic groups were decomposed. Sodium and chlorine in the brown coal were almost completely removed below 250 and 300 degrees C, respectively, during HTT. Around 20-30% of Ca and Mg in the brown coal was also removed by HTT below 350 degrees C. The viscosity of the coal solvent slurry prepared from a HTT coal was less than 1/10 of the viscosity of the coal solvent slurry prepared from a just dewatered coal. This enabled the increase of the coal concentration of the coal solvent slurry from 28 to 40 wt %. The significant increase of the coal concentration of the coal solvent slurry prepared from the HTT coal was found to be realized mainly by the decrease of the pore volume of the coal particles through HTT. These results suggest that HTT will be a pretreatment method effective not only to increase the space time yield but to suppress the scale formation of the brown coal liquefaction process.