To elucidate the dependence of the optical properties of semiconductor nanoparticles (NPs) on their size, it is of requisite importance to control or narrow the size distribution of NPs. In this paper, we demonstrated how to attain such controlled states by classifying manganese-doped zinc sulfide (ZnS:Mn) NPs. The classification method proposed was the gradual stage-wise flocculation of NPs relying on interparticle interaction forces between NPs, which were controlled by the addition of a poor solvent, 2-propanol. The mean diameter of ZnS:Mn NPs was evaluated from X-ray diffraction pattern and UV-vis absorption spectrum with the tight-binding approximation. As the number of classification cycles increased, smaller-sized particles were collected. The first-collected sample, which comprised a group of the largest-sized NPs, showed the highest photoluminescence intensity. (C) 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.