The effect of heavy boron doping on oxygen precipitation in Czochralski silicon substrates of epitaxial wafers has been studied with transmission electron microscopy observations and a preferential etching method. Prolonged isothermal annealing between 700 and 1000 degrees C for up to 700 h was performed on p/p+ (5-20 m Ohm cm) and p/p- (10 Ohm cm) wafers. It was found that, with an increase in boron concentration, (i) the precipitate density increased, and (ii) the precipitates could nucleate at a higher temperature. The growth process of platelet precipitates was also investigated and compared with the process in polished p- wafers. It was confirmed that (i) precipitate growth rate in p/p+ wafers was higher than that in p- wafers, and (ii) precipitate nucleation in p/p- wafers was delayed compared with p/p+ wafers. The precipitate growth in p/p+ wafers was determined to be reaction-limited, which differed from the diffusion-limited growth in p- wafers.