Quantized ZnS particles were prepared and isolated from aqueous solution with a capping agent, thioglycerol. The absorption peak of the particles was located at 259-260 nm, and the diameter was estimated to be 1.5-1.6 nm from X-ray diffractograms. Their excitation energy agrees well with that calculated with the finite depth potential well model. ESR spectra were measured for the quantized and bulk ZnS particles at 77 K under photoirradiation. For the quantized ZnS, only surface hole radicals were observed, while several kinds of radicals consisting of trapped electron, inner, and surface holes were observed for the bulk ZnS. A high activity of photocatalytic reactions for quantized semiconductor particles may arise from a large number of radicals photoproduced at the surface.