Enhanced ozonation degradation of atrazine(ATZ) with nano-ZnO(nZnO) as catalyst and the influences of the operational parameters have been investigated through semi-continuous experiments in this study. The results demonstrated that the combination of ozone(O_3) and nZnO showed an obvious synergetic effect and the ATZ degradation conformed to pseudo-first-order kinetics. An improvement of ATZ degradation efficiency by 41.8% and pseudo-first-order rate constant by more than a factor of four was obtained in the O_3/nZnO process after 5 min of reaction compared to O_3 alone. Meanwhile, the degradation efficiency of ATZ was gradually enhanced with increasing nZnO dosage and initial pH in the range from 3.0 to 8.0, and a higher amount of ATZ was degraded when the initial concentration of ATZ rose from 0.5 to 5 mg/L. Additionally, sulfate ion, chloride ion, nitrate ion and low concentrations of humic acid substances led to enhancement of the ATZ degradation. The notable decrease of ATZ removal efficiency observed in the presence of radical scavengers and the results of free radical tests indicated thatUOH is the dominant active radical species. The mechanism investigation demonstrated that the enhancement effect could be attributed to the introduction of nZnO,which could promote the utilization of O_3, enhance the formation of superoxide radical, and further accelerate the production of hydrogen peroxide and the generation of OH/O_2~-.