Striated microdefect distribution (swirl defects) are formed by the heat treatment in Czochralski‐grown silicon crystals. The striation observed by etching has two period components 1–2 mm and 300–500 &mgr;m along growth direction. The corresponding microdistribution of oxygen was examined by the scanning infrared absorption method with the collimated beam of 100 &mgr;m in diameter. The results show that the distribution with the period of 1–2 mm was only observed, but not the microdistribution with the period of 300–500 &mgr;m. It is proposed from the results that microdefects are originated at a high oxygen‐concentration region from the nuclei which were introduced in a striated pattern with the period of 300–500 &mgr;m during crystal growth. The nucleus is not interstitial oxygen itself.