The annihilation of grown-in defects by hydrogen annealing have been explained as the dissolution of oxygen precipitates, because it has been generally thought that grown-in defects in Czochralski silicon crystals are gigantic oxygen precipitates. However, it is necessary to re-examine the mechanism of the annihilation of the defects by hydrogen annealing, because recently it has been shown that the grown-in defects were voids of octahedral shape. In this letter, a simulation model is presented which describes the annihilation process of void defects by hydrogen annealing. ©1997 American Institute of Physics.