The influence of Nb contents on the high temperature strength and ambient ductility of cast high Al containing TiAl alloys are investigated with Nb content varying from 2at%-9at%. It shows that the tensile strength increases firstly then decreases with increasing Nb conternt. Alloy with 7at%Nb has the highest tensile strength at 900 ℃, about 587 MPa。These high strengths arise from high Nb solute in the γ-TiAl matrix leads to a high critical resolved shear stress(CRSS) of dislocation, reduction of the stacking fault energy and reduction of diffusivity. Al/Ti antisite defects, the hard orientation of the lamellar and tensile direction and the twinning may provide a substantial hardening of high Al containing cast TiAl alloys. While B2 increases with increasing the Nb content, may be the reason that ambient ductility decreases with increasing Nb content.