Scale deposition is a very troublesome problem for a long-term stable operation of a direct coal liquefaction plant. The scale reduction effect of hydrothermal treatment (HTT) for a brown coal liquefaction was investigated using a 0.1 ton/day process development unit (PDU). It was found that the amount of scale formed was reduced by half compared to non-treated coal when HTT coal treated at 325 degrees C was liquefied. This was because most carboxyl groups were decomposed and exchangeable cations, such as Ca and Na, precursors of the scale, such as CaCO3 and NaCl, were reduced during HTT. Furthermore, the formation of scale comprising Fe1-xS and SiO2 was also suppressed by HTT probably because of a decrease in the amounts of NaCl and CaCO3. Liquefying the HTT coal slightly decreased the oil yield compared to the non-treated coal. However, this disadvantage is compensated by the increase in the space time yield of the reactors liquefying HTT coal, because the coal concentration of the HTT coal-solvent slurry fed to the reactors can be increased from 28 to 42 wt % as a result of the reduction of viscosity, as reported in an our previous paper. The concentrations of major scale precursors in the HTT coal-solvent slurry of 42 wt % coal concentration are lower than those in the non-treated coal-solvent slurry of 28 wt % coal concentration. These results indicate that HTT is an effective pretreatment method not only to realize a long-term stable operation but also to improve the oil productivity of the liquefaction plant.