By means of the Empirical Electron Theory (E E T), the valence electron structures of ordered and disordered TiAl at various temperatures are calculated. The effect of crystal lattice expansion and decrease of long -and short-rang order parameter (with increasing temperature) on strengthening is analyzed. The reason of an anomalous yield behavior in TiAl is that the valence electron structures directly effected by temperature result in strengthening, and "disorder domain" strengthening (with short order -range parameter descending). According to this view point, an mechanism is proposed, and some experimental results that content and grain size have influence on anomalous yield behavior in TiAl are explained.