This book focuses on fabrication of ZnO/CdS optical sensor. Nanocrystalline ZnO/CdS core-shell optical sensors were fabricated by chemical bath deposition (CBD) method. Vertically aligned ZnO nanorods were firstly synthesized on silicon p-type (111) by CBD method with assist of microwave. The CdS were deposited on the ZnO nanorods to create core-shell structure by using CBD method with cadmium chloride (CdCl2) 0.15M as Cd2+ ion source at difference deposition time. ZnO nanorods were the core and CdS were the shell of this core-shell structure. The photoconductive testing was carried out to test this optical sensor light detection by shining ultraviolet (UV) light pulse on the devices. The devices affect the light absorption and have higher sensitivity to the light. The highest sensitivity reached 53% for the ZnO/CdS optical sensor while the ZnO optical sensor only 6.77% and ZnO/CdS can response to high frequency light pulse. The energy band gap of ZnO/CdS was also lowered to 3.3eV compared to ZnO, 3.38eV. These results demonstrate the existence of CdS has enhanced the ability and the properties of optical sensor compared to ZnO nanorods optical sensor alone.