A computer simulation model for oxygen precipitation during crystal growth and wafer annealing of Czochralski silicon is constructed. In this model, the precipitate morphology is determined by minimizing the excess free energy. This model is connected with the formation models of grown-in defects, such as crystal-originated particles (COPs) in the vacancy-rich region or extrinsic stacking faults in the self-interstitials rich region. That is, the simulation of oxygen precipitation starts at 1000 or 900degreesC with the input data of residual point defect concentrations after COP or stacking-fault formation. The model well simulates oxygen precipitation behavior. For example, the simulated results of isothermal annealing at 700 and 800degreesC show that (i) the precipitate morphology is initially spherical, then changes to oblate spheroidal with an increase in annealing time, and (ii) the aspect ratio beta is almost constant at (2.0-6.0) x 10(-2) for longer than 200 h. The simulated beta agrees approximately with experimentally determined beta = (0.8-2.5) x 10(-2) of platelet precipitates reported in the literature. The values of both the density and the growth rate of precipitates agree for the simulations and experiments. The model also shows that oxygen precipitation during crystal growth strongly depends on the residual point defect concentrations after grown-in defect formation. (C) 2003 The Electrochemical Society.