Abstract: The variability and corresponding mechanisms of the Arctic Oscillation (AO) during three typical periods, the Medieval Climate Anomaly (MCA), Little Ice Age (LIA), and Present Warm Period (PWP), in the last millennium were analyzed using simulations from nine Earth system models (ESMs) from the Paleoclimate Modeling Intercomparison Project Phase III (PMIP3) Last Millennium experiment and Paleoclimate Model Intercomparison Project Phase 5 (CMIP5) historical experiments. Compared with the NCEP reanalysis data, the ESMs reasonably reproduce the AO spatial pattern and inter-annual period, and most ESMs reproduce the AO strengthening trend in the last five decades. Simulations show that there is no consistent AO phase during the MCA among the different models. The eight models simulated generally negative AO phases during the LIA and positive phases during the PWP. These simulated results are consistent with previous studies using proxy reconstructions and observations. The multi-model ensemble mean indicates that there is no significant sea level pressure (SLP) change over the Arctic region during the MCA. The SLP anomalies over the Arctic region are significantly positive during the LIA and significantly negative during the PWP. These changes in SLP are related to the anomalous lower temperature during the LIA and higher temperature during the PWP over the Arctic region. Our study suggests that the AO variability during the LIA and PWP are influenced by the natural and anthropocentric forcing, respectively.