The as-cast TiAl alloys containing tungsten and niobium were exposed to 700 °C air for a long term and effects of the exposure on microstructures and mechanical properties were investigated. Results show that the γ phase is stable and the phenomena of recovery, recrystallization and disconnection are never observed during the long-term thermal exposure. However, the metastable α2 lamellar decompose to α2+γ phases, resulting in embrittlement of the alloys. The tensile and fatigue resistance of TiAl alloys are improved after “parallel decomposition” of α2 lamellar but the “perpendicular decomposition” of α2 lamellar is detrimental to tensile resistance. The heavy element additions can slow down the “perpendicular decomposition” of α2 lamellar.