We investigate the decay process of photoelectrons from a luminescent material of ZnO:Zn using a microwave dielectric spectrometry. Electrons in the conduction band are found to decay exponentially and the lifetime is 781 ns, while the time interval of decay from the maximum to half of this value is 470ns. ZnO:Zn is a green luminescent material at its central wavelength of 510nm. Compared to the decay of electrons in the conduction band, the decay process of the luminescence is faster, and the time interval of decay from the maximum to half of the maximum is about 100ns. We believe that the mechanism of the ZnO:Zn visible luminescence is recombination luminescence, and find that our theoretical simulation is in agreement with the experimental results.