To clarify the oxygen precipitation factors in silicon crystals, the oxygen precipitation behavior according to heat‐treatments in a whole Czochralski‐grown silicon ingot, from the seed end to bottom end, is examined using the infrared absorption method and TEM. Comparison of results shows that oxygen precipitation (i) strongly depends on the initial oxygen concentration , (ii) rarely occurs in wafers with less than , (iii) proceeds most rapidly during the annealing at 1000°C, and (iv) is contingent on the specimen position in the crystal ingot. The results of (iii) and (iv) are supported by TEM observation, which reveals the densest dislocations in the specimen annealed at 1000°C and the difference in oxygen precipitates between the specimens with the same but taken from different positions in an ingot.