We previously demonstrated that a swept-source optical coherence tomography system (SS-OCT) without polarization-sensing could quantitatively estimate the progress of cavitated dentin lesions in vitro. The purpose of the current study was to investigate the ability to estimate the lesion development in non-cavitated dentin lesions with a surface layer. Bovine root dentin specimens were subjected to different periods of demineralization (1 to 14 days). Cross-sectional images of the specimens before and after the demineralization were captured by SS-OCT at 1310 nm center wavelength. Following each period, the specimens were cut into sections for transverse microradiography (TMR) and correlations between SS-OCT data and TMR parameters were examined. TMR images of the specimens showed subsurface lesions ranging 100 to 400 μm in lesion depth (LD) and 1,000-5,000 volume in mineral loss (ΔZ). SS-OCT images showed a boundary suggesting the lesion front. Reflectivity increased with demineralization progress. A strong correlation was found between the boundary depth from the lesion surface and LD. Integrated dB values from the lesion surface to the boundary depth before and after the demineralization (R S and R D, respectively) were calculated. There was a significant positive relationship between ΔZ and R D; but not between ΔZ and ΔR, where ΔR=R D-R S. A clear indication of surface layer thickness was not found on OCT images. In conclusion, SS-OCT system could provide quantitative estimation of caries progress in artificial subsurface dentin lesion in terms of lesion depth. Further studies are needed to investigate whether other parameters of the dentin lesion with surface layer could be determined by this technique. Study supported by Global Center of Excellence at TMDU and NCGG.