Thin films of cadmium sulfide (CdS) were prepared with ammonium chloride, cadmium chloride, potassium hydroxide and thiourea by chemical bath deposition (CBD). For comparison, CdS films were also deposited by radio frequency (RF) magnetron sputtering, using CdS and argon as a target and reactive gas, respectively. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible spectroscopy. The results show that the CdS films prepared by the above two methods have (002) orientation, the CdS films deposited by RF magnetron sputtering are more compact and much smoother than those prepared by CBD, and have lager crystalline size of about 20~30 nm. The CdS films prepared by CBD have smaller crystalline size and more defects. The properties of CdS thin films prepared by RF magnetron sputtering are totally superior to those of CdS films by CBD, but the optical transmittance of CdS thin films at short wavelength is an exception. The energy gap of CdS films prepared by the two methods are all in the range of 2.3~2.5 eV.