Early dental caries is caused by demineralization of the teeth, which results in the increasing backscattered coefficient of the lesion area. In the paper, a method is proposed for detecting early caries by using optical coherence tomography (OCT). The two dimensional tomography images of the human teeth in vitro are obtained by using the development all fiber OCT system. Artificial caries model is made by chemical etching on the human teeth in vitro for 12, 24, 48, 72, 96 and 120 h, respectively. Using the OCT system, the level of demineralization is obtained quantitatively and the measurement data are obtained with different artificial time. The result shows that the depth of the artificial caries has the linear relationship with time. It is proved that OCT has the feasibility by detecting the level of demineralization for early caries diagnosis.