Microstructure evolution of Ti-45 Al-8. 5 Nb-( W,B,Y) alloy exposed to isothermal processing were investigated using dilatometry( DIL),optical microscopy( OM),X-ray diffraction( XRD) and microhardness tester. The results indicate that the transformation temperature of α_2→α is 1140 ～1209 ℃ and the growth curve of α phase is"S"shape during isothermal treating. The microstructure of this alloy is lamellar and it is consisted of α_2,γ and a little of βphase. β phase at the grain boundary gradually decreases with the holding time increasing and the size of γ phase generated along the grain boundary increases,however,the microhardness of alloy increases firstly and later decreases with the increase of holding time.