Abstract: The method for preparation quinacridone pigment nanocrystals with sub micrometer to nanometer in average size comprising, supplying supercritical or semi-critical liquid, which dissolves quinacridone pigment, into a specimen tube (ST) composing a reactor possessing a filter of desired opening to the upper stream side and to the down stream side in which quinacridone pigment is set up, flowing out the supercritical or semi-critical liquid in which quinacridone pigment is dissolved from said reactor to a mixing apparatus to which coolant is supplied, and by selecting the kind of supercritical or semi-critical liquid and combination of conditions e.g. supplying speed of each liquids, temperature of the liquid and the reacting pressure.