Mesocyclops leukarti of zooplankton propagates excessively in eutrophic water body and it cannot be effectively inactivated by the conventional drinking water treatment process. In order to tackle this problem, a study of removal effect on Mesocyclops leukarti with chlorine dioxide in a waterworks was performed. The results showed that Mesocyclops leukarti could be effectively removed from water by 1.0 mg/L chlorine dioxide preoxidation combined with the conventional drinking water treatment process. Higher oxidizability and molecular state of chlorine dioxide in water is the key to the inactivation of Mesocyclops leukarti. The chlorite, disinfection by-products (DBPs) of chlorine dioxide, was stable at 0.45 mg/L, which is lower than that critical value of the USEPA. GC-MS examination showed that the quantity of organic substance in the water treated by chlorine dioxide obviously decreased. Ames test further revealed that the mutagenicity was reduced by chlorine dioxide with respect to prechlorine. The propagation of Mesocyclops leukarti can be inactivated effectively and safely by chlorine dioxide pre-oxidation. Mesocyclops leukarti of zooplankton propagates excessively in eutrophic water body and it can not be effectively inactivated by the conventional drinking water treatment process. A result of a study of removal effect on Mesocyclops leukarti with chlorine dioxide in a waterworks was performed. showed that Mesocyclops leukarti could be effectively removed from water by 1.0 mg / L chlorine dioxide preoxidation combined with the conventional drinking water treatment process. Higher oxidizability and molecular state of chlorine dioxide in water is the key to the inactivation of Mesocyclops leukarti. The chlorite, disinfection by-products (DBPs) of chlorine dioxide, was stable at 0.45 mg / L, which is lower than that critical value of the USEPA. GC-MS examination showed that the quantity of organic substance in the water treated by chlorine declination . Ames test further revealed that the mutagenicity was reduced by chlorine dioxide with respect to prechlorine. The propagation of Mesocyclops leukarti can be inactivated effectively and safely by chlorine dioxide pre-oxidation.