The annealing properties of defects in BF2 + implanted silicon were investigated using positron annihilation, TEM and SIMS techniques. For the as-implanted specimens, the major species of defects was identified to be divacancies. After thermal annealing, vacancy-fluorine complexes were formed. The size of open volume in the vacancy-fluorine complexes became larger with increasing annealing temperature. Even after 1100 °C , the vacancy-fluorine complexes were still stable with the size of open volume close to V5. The depth profile of vacancy-fluorine complexes was not largely changed after re-crystallization. In this way, the fluorine atoms played an important role in forming the defects with a large size of open volume. After 1100°C annealing, the fluorine atoms piled up at two regions; the projected range of fluorine and the original amorphous/crystalline interface, where bubbles were observed by TEM. The vacancy-fluorine complexes detected by positron annihilation might be precursors of the bubbles observed by TEM.