The morphology and growth process of oxide precipitates in Czochralski silicon have been studied with prolonged thermal treatments up to 700 h at intermediate temperatures (700-900-degrees-C). It was found with transmission electron microscopy observation that (i) the morphology of precipitates changes from platelet to aggregation of polyhedra at both 800 and 900-degrees-C during isothermal heat treatment, and (ii) the growth of platelet precipitates follows a t1/2 law.