Experiments with ZnO Metal-Oxide-Semiconductor (MOS) under different circumstances were made to get four different I-V curves. There were four conditions: dark, and tests with the green, blue, ultraviolet LED light. According to references, three parameters B,VB0and Nbarrcould be acquired by fitting lines of the I-V curves using MATLAB and LabVIEW. From their definitions, B, VB0and Nbarrindicate photoelectric properties of ZnO cooperatively under concrete conditions. VB0,grain boundary potential, is parameter of extrinsic properties of ZnO determined by both ZnO and testing conditions. So VB0is critical to control the photoelectric properties of ZnO. A smaller VB0, the stronger the photoelectric response of ZnO and the lager the efficiency of photoelectric conversion. Besides, this theory can be expanded to test the photoelectric properties of the other semiconductor materials. And I-V curves can direct the application of these materials efficiently.