A novel preparation method of CdS nanoparticles with controllable size and stability is presented. Poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) and cadmium ions form aggregates of single micelles called compound micelles, upon addition of the cadmium acetate salt into a solution of the block copolymer in tetrahydrofuran. The growth of CdS nanoparticles is confined to the core of single micelles after introduction of hydrogen sulfide gas into the solution. UV-visible spectroscopy, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the prepared nanoparticles. UV-visible absorption spectra show that larger nanoparticles are produced at lower 2VP:Cd-2 molar ratio. UV-visible absorption spectra and fluorescence spectra both indicate that with a decrease of block copolymer concentration in tetrahydrofuran, the size of the CdS nanoparticles decreases. The stability of the CdS nanoparticles and micelles was also investigated. UV-visible results show that the prepared CdS nanoparticles have good colloidal stability.