Behavior of oxygen, carbon and minority-carrier lifetimes of multi-crystalline silicon (mc-Si) has been investigated by means of FTIR and QSSPCD after three step annealing. For comparison, the annealing of czochralski (CZ) silicon was also carried out under the same conditions. The results revalted that oxygen and carbon concentration pf mc-Si had a larger decrease than that of CZ -Si, which means the more oxygen precipitates in mc-Si were generated. High density defects of mc-Si such as grain boundaries and dislocations accelerated formation of oxygen precipates and defect complex compounds are generated after three step annealing, which could be suction center of defect, that reduced carrier decentralized recombination centers that resulted in improvement of lifetime of wafer. Tendency of difference of lifetime was correlated with interior structure of crystalline silicon.