microstructure differences between the polysynthetically twinned crystal and the directional lamellar structure prepared by conventional casting for TiAl based alloy were analyzed, in order to detail discuss their influence on the microstructural degradation during creep loading. And micro-alloying methods were proposed to decrease the adverse effect. The microstructural differences mainly included colony boundary, Al segregation and excess α2 phase. Additionally, equiaxed gamma grain precipitation and lamellar coarsening during hot isostatic pressing were also other differences. These differences had adverse influence on the microstructure degradation of directional lamellar structure prepared by conventional casting during creep loading. Al segregation and excess α2 phase promoted the microstructural degradation for as-cast sample, and the equiaxed gamma grain precipitated during hot isostatic pressing aggravated the microstructural degradation. They caused significant reduction of creep rupture life. The effect of the former was less than that of the latter. The colony boundary had a little adverse effect on the microstructural degradation. Additionally, the methods that minor addition of Zr,Si,C improved the creep rupture properties by inhibiting microstructural degradation were also discussed.