By stochastic search and the first-principles calculations, we have carried out a systematic investigation on the structural stabilities and electronic properties of sulfur-modified diamond nanocrystals. Among the possible catenarian, annular and cage-like candidates, we determine the stable structures as a function of hydrogen/sulfur chemical potentials according to the phase diagrams. In addition, we also study the electronic properties of sulfur-modified nanocrystals, including the gap modulation and charge distributions.