ZnO Quantum dots with particle sizes about 5 nm were prepared by sol-gel method, the research about the photo luminescence(PL)/photoluminescence excitation(PLE) spectrum and the fluorescence lifetime indicated that the green emissions can be divided into two parts which were attributed to different transit mechanisms. The higher energy green emission was due to the electrons transit from conduction band to oxygen vacancies while the lower energy green emission was assigned to the electrons transit from the shallow donor levels which enhanced the green emission to the oxygen vacancies and the shallow donor levels was also the reason why the green emission reach strongest intensity when the excitation energy was a little lower than the band gap energy. While the blue emission can be explained by the electrons transiting from Zinc intervals to the valence band. The two mechanisms of green emission are first highlighted and should be considered in the optical application.