A direct coal liquefaction plant was constructed in Kashima, Japan, based on the concept of "NEDOL Process", and successfully processed 150 tons of Tanitorarum coal per day during 1997-1998. The plant was equipped with three reactors: 1 m in diameter and 11.8 m in length, connected in series. During the operation, solid particles were accumulated mainly in the first reactor. Slurry samples were directly removed from the reactors, and size distributions of solid particles were determined. Two types of particles were found: particles with cares and particles without cores. The size of the former particles was in the range of 10-200 mu m, while that of the latter particles was 1-80 mu m. The growth rate of the particles was estimated to be 0.10 nm s(-1) under the reaction conditions. The solid accumulation in the first reactor was classified into a dense region in the lower part of the reactor and a lean region extending above the dense region. The former region was expressed as a three-phase fluidized bed model, and the latter was expressed by a one-dimensional sedimentation-dispersion model. These reactor models were validated on the basis of pressure differences and particle size distributions. It was confirmed by the simulation that removing a small amount of solids from the bottom of the first reactor was effective to achieve a long-term continuous operation without accumulation of coarse solid particles.