An electron irradiation induced fast phase-separation behavior was observed under convention Transmission electron microscopy (TEM) observation of spark plasma sintered AlF₃ doped alumina ceramic. Spherical nanocrystalline Al precipitates separated out from original alumina grain surface within several seconds under transmission electron microscopy electron irradiation. By high resolution TEM observation combined with diffraction patterns analysis, it was found that the original alumina grain surface was in highly defected state. After electron irradiation under TEM, the defects on original alumina surface vanished accompanied by the precipitation of nanocrystalline Al particles. By thoroughly analysis of the defect reaction during doping process and the feature of cation sub-lattice of alumina, a defect assisted interstitial atom segregation mechanism was proposed to explain this behavior. According to this mechanism, doped F ions first occupied oxygen vacancy sites with corresponding Al ions at intrinsic interstitial sites. After oxygen vacancies being fully occupied, both F and Al ions tended to settle down at intrinsic octahedron interstitial sites, which resulted in a metastable doping state. Under the act of 1/3 [11ˉ00] partial dislocation of alumina matrix, distorted cation sub-lattice generated double aggregated vacant octahedron sites. When these doublets vacant octahedron sites were occupied by foreign Al ions, stacking faults composed of about three sequences were generated as that observed in high resolution TEM. Meanwhile, the segregated doping Al ions at double aggregated octahedron sites along the stacking faults worked as early stage precipitations. Under electron irradiation, with the ablation of F ions, the unstable segregated Al ions separated out as nano precipitation with the reconstruction of alumina lattice.