Abstract: Using the IAP two一level general circulation model, the ice age July climate was simulated through the surface conditions of 18,000 years before present assembled by CLIMAP. Comparing with the present July simulation results, the ice age atmosphere.is found to have a substentially lower temperature, precipitation and cloudness, higher sea-level pressure, especially at high latitude land regicn of Northern Hemisphere and Antarctic. The ice age climate would have been changed little if the CO2 content is set at modern value, which shows the problem of CO2 sensitivity should be studied by means of coupled models. It was also pointed out that there are some common characteristics between CO2 induced climatic changes and the ice age surface condition-induced climatic changes, which may enlighten our understanding of how climate system responds to external forcings.