Photoluminescence in the photon energy range 0.7–1.2 eV is investigated for commercial Czochralski‐grown Si crystals subjected to isothermal annealing at 650 °C for 1–450 h. A strong and sharp line appears at 0.903 eV at certain annealing stages, regardless of the sources and the conductivity type of starting materials. The intensity dependence of the 0.903‐eV line on the annealing time coincides with the concentration dependence expected for Si self‐interstitials emitted during the oxygen precipitation process. This is experimental evidence that microdefects responsible for this line are correlated with Si self‐interstitials.