Time-resolved photoluminescence (PL) of ZnO epitaxial thin films has been measured by using an up-conversion method at room temperature. The time-integrated PL spectrum has one dominant peak at similar to 3.14eV which originates from electron-hole plasma (EHP). The rise and decay times of the PL strongly depend on the monitored photon energy. The observed photon energy dependence of the time-resolved PL are thought to be due to the recombination of hot carriers during the cooling process, the accumulation process of the carriers from the bottom of the band and the radiative recombination of EHP state.