The photoluminescence (PL) technique has been used in the analysis of heat‐treated silicon crystals to investigate the origin and the behavior of thermally induced defects. The PL measurements were made at liquid helium and liquid nitrogen temperatures on various groups of commercial wafers which were subjected to an isochronal annealing at a temperature between 450 and 1250 °C for 64 h. The PL results were compared with the results obtained by other characterization techniques, in particular, the infrared (IR) spectroscopy. After heat treatment at 450 °C, a new PL pattern appeared at 4.2 K, if the crystal contained a high concentration of oxygen. The new pattern is proved to be associated with the thermally induced donors. The PL and IR results classified Czochralski‐grown wafers into two categories. Various heat treatments made a great change in the PL spectra for some wafer groups (category I), but a little change for the other wafer groups (category II). Correspondingly, the decrease in the interstitial oxygen concentration due to annealing was large for category I and small for category II. The PL intensity reduction at lower temperatures (600<T<900 °C) was closely correlated with the formation of silicon‐oxygen complexes. It can be concluded that the Si‐O complexes is produced with greater ease in category I than in category II. The variation in the configuration and/or the density of the Si‐O complexes may induce the PL spectral change. Possible factors which separate the two categories are residual carbon and the thermal history of the crystal.