We report on an investigation to the effects of supercritical solution concentrations on the crystal morphology, crystallinity, growth rate, and number density of anthracene grains in thin films on silicon substrates by rapid expansion of supercritical solutions (RESS) using carbon dioxide to examine the crystal growth mechanism of anthracene thin films. The solution concentrations varied in the range of 2.10-7.15 x 10(-5) (molar fraction) at an equilibrium temperature of 318.2 K and an equilibrium pressure of 15.0 MPa and in the range of 1.72-6.34 x 10(-5) (molar fraction) at an equilibrium temperature of 328.2 K and an equilibrium pressure of 15.0 MPa. The morphology of the grains was island-like at higher solution concentrations and was dendritic or fractal at lower solution concentrations. The growth rate of dendritic grains was higher than that of island-like grains and the trend of the growth rate changed at a concentration of 3.40 x 10(-5) (molar fraction; a threshold concentration) with a change in morphology of grains from dendritic to island-like. Although the number densities of island-like and dendritic grains increased almost linearly with the solution concentration, the trend in the number density of grains also changed at the threshold concentration, with the change in morphology from dendritic to island-like grains. The crystal growth of grains deposited on the substrate was dominant and most of the given supersaturation was consumed for the crystal growth of grains at solution concentrations lower than the threshold concentration. At solution concentrations higher than the threshold concentration, the crystal nucleation on the substrate was dominant and most of the given supersaturation was consumed for the crystal nucleation of grains, followed by the crystal growth of the grains.