Processes involving the thermal activation energy and the activation energy of the illumination are shown to be of basic importance for the photoconductivity properties of CdS powder and sensitized CdS powder. Sensitizing by dye in CdS decreases activation energy in the illumination compared with thermal activation energy and increases the true photosensitivity. Trap depths in vacuum for the CdS powder and sensitized CdS is observed by the measurement of the thermally stimulated current (T. S. C.). The experimental results of T. S. C. indicated that the trap depths of CdS and sensitized CdS recognized the exsistence of the various traps which was made to fill the electrons from the shallow trapping levels to conduction band. It is interpreted by injecting thermally electrons in sensitized region from the number of the traps to conduction band of the CdS in the room temperature.