The B2 (FeAl) antisite defects and microstructure evolution during homogenous transformation of Fe-Al alloys are investigated by phase field kinetic model considering long-range elastic interaction energy. The results show that congruent ordering transformation dominates the aging initial stage, and two phase structures of B2 ordered phase and disorder phase are formed. The antisite atom occupation probability which indicates antisite defects decreases during congruent ordering transformation of aging initial stage. With increasing aging time, spinodal decomposition occurs within B2 structure ordered phases, and spinodal decomposition and ordering coexistence microstructures are formed at this stage. At the aging of final stage, the increase of antisite atom occupation probability is attributed to the presence of spinodal decomposition. For Fe-Al alloys with different composition, we find that with increasing Al atom concentration, the Al(Fe) antisite defects increase, and the Fe(Al) antisite defects decrease. In contrast, with the increase of Fe atom contents, the numbers of Fe(Al) antisite defects increase and that of the Al(Fe) antisite defects decrease.