Formation of micronuclei and binucleate cells and their relationships with cell survival were studied in the aerobically- and anaerobically-irradiated 3T3 cells. The results showed that frequency of micronuclei, percentage of micronucleus cells and percentage of binucleate cells increased linearly with the radiation dose in certain range. Oxygen enhancement ratios (OER) of micronucleus frequency, percentage of micronucleus cells, percentage of binucleate cells and cell survival were 2.02,1.96, 1.87 and 1.83, respectively. The percentage of micronucleus cells or percentage of micronucleus cells plus binucleate cells correlated negatively well with cell survival. The mechanism of oxygen effect in the radiation response of 3T3 cells and the significance of formation of micronuclei and binucleate cells were discussed.