ZnO nanostructures were synthesized on Si and Si/SiO2 substrate by well established thermal-evaporation-deposition method which involves vapor-solid growth process for non–catalysts activated growth. Scanning electron micrograph shows difference type of ZnO nanostructures have been synthesized. Electrical I-V characterization was measured by using Al as electrode at room temperature. Schottky contacts were obtained for both contact of ZnO nanowires and Al. A symmetric metal-semiconductor-metal junction was obtained. A two-opposit-diode equivalent circuit was applied to explain this I-V characteristic.