We report on an investigation to the e ects of supercritical solution concentrations on the crystal morphology, crystallinity, growth rate, and number density of anthracene grains in thin lms on silicon substrates by rapid expansion of supercritical solutions (RESS) using carbon dioxide to examine the crystal growth mechanism of anthracene thin lms. (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.