This article reports on a study of the photoluminescence (PL) characteristics of SiO2/Si/SiO2 quantum well structures formed by the thermal oxidation of Si lines with rectangular cross sections and (111) side planes. The PL spectra have a peak at a wavelength of 760 nm regardless of Si width, indicating that the emission is due to the recombination of carriers at the Si/SiO2 interface. The Si width required to obtain the maximum emission is about 5 nm, which is thicker than theoretically predicted. The reason for this is clarified by measuring the difference in the PL characteristics for patterned and unpatterned SiO2/Si/SiO2 film structures. In the film structures, the characteristics are found to vary by the patterning of the ultrathin Si layer, followed by oxidation; the Si thickness yielding the maximum intensity increases and a wide range of Si thickness contributes to the emission. This behavior is consistent with that of the oxidized Si lines and is possibly due to pattern deformation caused by oxidation. The stress accompanying the deformation is probably the reason for the increase in the PL intensity in thicker Si. Therefore, the PL characteristics for Si are strongly influenced by the patterning and subsequent oxidation.