Flower-like CdS nanostructures consisting of sword-like nanorods have been prepared via organic-free hydrothermal method using thioacetamide (TAA) as sulfur source. X-ray diffraction pattern (XRD) shows that the obtained flower-like CdS nanostructures are of hexagonal phase. The selected area electron diffraction (SAED) pattern identifies that the flower-like CdS nanostructures are single crystalline in nature. Furthermore, the optical properties of flower-like CdS nanostructures have been characterized by ultraviolet–vis (UV–vis) and photoluminescence (PL) spectra. Finally, the investigation on the mechanism indicates that the internal structure and sulfur mass transport controlled by TAA play the critical role in the formation of flower-like CdS nanostructures.