The shape of the oxygen precipitate in Si wafers generated by thermal annealing takes various forms depending on thermal annealing time and temperature. In this experiment, we observed, in detail, the shape of the oxygen precipitate in a Ct-Si sample after two-step annealing at different thermal annealing times by transmission electron microscopy (TEM). We found that the precipitate shape changed from octahedral to polyhedral to octahedral. Moreover, a lot of punch-out dislocations were generated with this change. This shape change was considered from the viewpoint of thermodynamics.