Four kinds of microstructural instability of a TiAl-based alloy during rapid heating cyclic heat treatment were observed: (i) the formation of secondary lamellae, (ii) the spheroidization of primary lamellae, (iii) a discontinuous coarsening at grain boundaries and (iv) nucleation at phase interfaces. By comparing the starting line of continuous heating transformation with that of TTT diagram, the similarity and difference of the microstructure instability during continuous heating and isothermal heating conditions were studied. Ways of avoiding unfavorable microstructural instability were proposed such as: (i) insuring homogeneity of the composition so as to guarantee the identity of transformation temperature in the alloy; (ii) holding at the highest temperature for as short a time as possible.