CdS nanocrystals including nanorods and faceted nanoparticles were prepared via thermolysing cadmium ethylxanthate in hot hexadecylamine (HDA) solution. In order to control the shape and size of the nanocrystals, the influence of reaction conditions (such as reaction temperature, monomer (single-source precursor—cadmium ethylxanthate) concentration and reaction time, etc.) on the morphology, the surface states and the dispersivity of the CdS nanocrystals was comprehensively investigated. The CdS nanocrystals were characterized by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), absorption spectroscopy and photoluminescence spectroscopy. It was found that under the conditions of high monomer concentration and adequate reaction time (3 h), long multi-armed nanorods were produced dominantly when the reaction temperature was not higher than 200 °C, while at high temperature (260 °C), the formation of short single-armed nanorods was preferred. When the monomer concentration was low and the reaction time was much longer (such as 12 h), spherical-shaped nanocrystals were mainly obtained, while if the reaction time was between 4–6 h, faceted nanocrystals were produced. The mechanism of the effect of the reaction conditions on the shape and size of the nanocrystals is discussed. The variation of reaction conditions can shift the reaction system between thermodynamic and kinetic control, which determines the morphology of the resultant nanoparticles. The formation of anisotropic rod-like CdS nanocrystals was found to be a highly kinetics-driven reaction, and a high monomer concentration in the reaction environment is a necessary condition for synthesizing rod-like CdS nanocrystals. By optimal combination of the reaction temperature, monomer concentration and reaction time, size- and aspect ratio-tunable one-dimensional (1D) CdS nanorods and hexagonal-like CdS nanocrystals which self-assembled into supperlattice arrays were obtained.