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The effect of thermal exposure at 600-750℃ for 300h in atmosphere on room temperature tensile ductility of cast Ti-47.5Al-2.5V-1.0Cr-0.2Zr(atom fraction/%) was investigated.The results indicate that no surface layer on the specimen surface is observed after exposure at 600℃,but tensile ductility at room temperature decreases from 2.81% to 2.54%,the reduction is only 9.6%;after exposure at 650℃,a Al depleted layer is formed on the surface of substrate,tensile ductility is further reduced to 1.54%,the reduction reaches 45%. When exposure temperature increases to 700℃,tensile ductility does not continuously decrease due to no significant increase in the thickness of the Al depleted layer;when the exposure temperature increases to 750℃,tensile ductility is further reduced to 1.12% due to remarkable increase in the thickness of Al depleted layer with the reduction of 60%.Tensile ductility at room temperature decreases in different degrees as the exposure temperature increases,which is attributed to emerging and step-increase of the Al depleted layer adjacent to the substrate, and is not related to the formation of outside oxide layer and the changes in thickness.