A synthetic study is made on the global or hemispheric mean temperature series for the last millennium worked out by Mann et al., Jones et al,, Crowley and Lowery, and Briffa. The global mean temperature series reconstructed by using proxy data at 30 sites by Wang et al. and simulations from AD 1000 to 2000 by energy balance model are described and compared with the series of others. Wang’s series gives greater variability and shows the highest correlation coefficient (0. 83) with the simulation results. Uncertainties in the reconstructions and simulations are discussed. The errors in reconstructing a global mean temperature series according to 30 sites as used in the research are estimated. Wang s series indicate that temperature average for the 11th century is higher than the mean of the last millennium. It infers that the Medieval Warm Period predominated to some extent over the globe. However, the 20th century is no doubt the warmest century during the last millennium. A synthetic study is made on the global or hemispheric mean temperature series for the last millennium worked out by Mann et al., Jones et al ,, Crowley and Lowery, and Briffa. The global mean temperature series reconstructed by using proxy data at 30 sites by Wang et al. and simulations from AD 1000 to 2000 by energy balance model are described and compared with the series of others. Wang’s series gives greater variability and shows the highest correlation coefficient (0.83) with the simulation results. Uncertainties in the The errors in reconstructing a global mean temperature series according 30 sites as used in the research are estimated. Wang s series indicate that temperature average for the 11th century is higher than the mean of the last millennium. It infers that the Medieval Warm Period predominated to some extent over the globe. However, the 20th century is no doubt the warmest century during the last millennium.