A method was developed to measure low level oxygen concentration in thin epitaxial silicon film (epi film) by measuring photoluminescence (PL) intensity of an oxygen-carbon complex (C center) formed in silicon crystal by carbon implantation (energy: 20 keV). A calibration curve between the PL intensity of the C center and the oxygen concentration of standard samples for each carbon implantation fluence was obtained. In order to measure the oxygen distribution of the depth direction in an epi sample with the epi thickness of 1 mu m, many epi samples were etched to appropriate depths prior to the carbon implantation. The calibration curve established above was applied to the pi, intensity of the etched and implanted epi sample, and the relationship between the oxygen concentration in the epi sample and the depth from the epi-surface was obtained with the sensitivity level of 1 x 10(15) atom/cm(3). The concentration of oxygen, diffused from the substrate,increased monotonously from the epi layer to the substrate without any abrupt change at the interface between them. The oxygen depth profile of the whole region (epi layer and substrate) could not be fitted by a simple diffusion function such as a complementary error function.