Oxygen precipitation (OP) and annihilation of voids in heavily phosphorus (P)-doped Czochralski (Cz) silicon have been investigated. It was found that the nucleation anneal at 650°C resulted in much more pronounced OP in the subsequent high temperature anneal than that at 800 or 900 °C. This was due to that SiP precipitates could be formed in heavily P-doped Cz silicon by the 650oC anneal and they acted as the heterogeneous nuclei for OP in the following anneal at high temperatures. The rapid thermal anneal (RTA) at 1200°C was proved to be an effective means to annihilate voids. Moreover, it was found that the significant OP resulting from the two-step anneal of 650°C/8 h + 1000°C/16 h could also cause the substantial annihilation of voids in heavily P-doped Cz silicon. The mechanisms for the annihilation of voids have been tentatively discussed.