During Cenozoic, Earth's climate has undergone a progressive cooling, from bipolar ice-free ( greenhouse) to bipolar icesheets (icehouse), experienced several icesheet growth and decay events. At the Eocene-Oligocene boundary (E/O), δ¹⁸O increased rapidly from 1.2‰ to 3.0‰ in a short time, deep-sea temperature decreased from 12℃ to 4.5℃. The marine and terrestrial records show that near the E/O, global temperature has a significant decreasing, the biotic extinctions happened both in ocean and land, indicating the cooling and drying trend on global climate. During the Eocene-Oligocene transition, the Tasmania Seaway (between the Antarctic and the Australia) and the Drake Passage (between the Antarctic and South America) opening and widening, it may caused the formation of the Antarctic Circumpolar Current and the subsequent thermal isolation of the Antarctic continent, which formed the East Antarctic Ice Sheet. The last research finds that the E/O event is an instantaneous climate change, which has a close relationship with the atmospheric CO₂ concentration viriation. Its change rate similar with the atmospheric CO₂ concentration diversification reduced by humankinds activity at present, implying that the atmospheric CO₂ concentration viriation play a more important role than the opening of the passages in the E/O event.