Ab initio total-energy calculations were used to investigate the behaviors of CiCs and CiOi defects in Si and Si1-xGex alloys. It was found that in Si1-xGex alloys with different Ge contents, both CiCs and CiOi defects have good stability, and direct Ge—C or Ge—O bonds are unlikely to be formed. The A and B type structures of CiCs defects have similar characters in the alloys. The formation energy of A type configuration decreases with increasing Ge content. But the formation energy of B type decreases at first and then increases with the Ge content. The energy differences of A and B type configuration defects are between 0235 and 0220eV with lower Ge contents, and decrease sharply to around 01eV for higher Ge contents. The formation energy of CiOi pair displays a complex feature. Under isothermal annealing at 1000K and above, the structure of A type CiCs defect in pure Si was destroyed and transformed to B type defect. During annealing, the behavior of A type CiCs defect in Si1-xGex alloys depended on the annealing temperature, Ge content in the alloy, and the sites of Ge atoms.