We report studies of the effect of hydrothermal treatment on physical properties such as crystalline phase, size, and morphology of nanosized cadmium sulfide (CdS) particles. CdS precipitates have been synthesized by the reaction of Cd(NO 3 ) 2 with Na 2 S at room temperature. These CdS precipitates have been hydrothermally treated in the range 120-240 β’ C with variation of the treatment time. The effects of acid catalysts and other additives were also investigated. The particles prepared were characterized by XRD, TEM, and BET methods. With increased hydrothermal treatment temperature and time, crystallization from amorphous to crystalline form, cubic or hexagonal, and an increase of particle size occurred. CdS particles of well-developed hexagonal form were obtained at a hydrothermal treatment temperature of 240 β’ C; the primary hexagonal grain size was on the order of 20-30 nm. The addition of an acid catalyst, HCl, or of Cd(NO 3 ) 2 into the precipitate sol promoted crystal growth and phase transformation during the hydrothermal treatment, but another additive, Na 2 S, showed the opposite trend. It appears that hydrothermal treatment combined with proper additives could be an effective method for preparation of nanosize crystalline CdS particles.