Abstract： Objective To evaluate the effects of acid etching time on the degradation of type Ⅰcollagen in dentin. Methods Dentin was conditioned with 37% phosphoric acid for 10, 15, 30 and 60 s.There was no treatment for the control group. Quantity of collagen degradation in each group was determined by enzyme linked immunosorbent assay. Observations were carried out by means of a field emission in-lens scanning electron microscope (FEISEM). Results Samples conditioned with 37% phosphoric acid for 60 s showed the most degradation of collagen, which was 4. 86 (1.55) mg/g, followed by 30 s group and 15 s group, which were 2.76(0.87) mg/g and 1.93(0.88) mg/g, respectively. Group of 10 s was 0.95(0. 38) mg/g. The control group showed the least degradation of 0. 06(0. 03) mg/g. Significant differences in collagen degradation were found among groupo (P ＜ 0. 005). Smear layer were removed well but tubular orifices and collagen fibrils were covered by particles after dentin being etched with 37% phosphoric acid for 10 s, while open and clear tubular orifices were observed for 15 s group. Smoother surfaces of exposed collagen fibrils and fewer globular particles were found in 30 s group than in 15 s group. In the 60 s group,the number of major fibrils decreased while minor branching fibrils increased, which indicate that the intratubular structure collapsed and fibrils fractured. Conclusions Dentin conditioned with 37% phosphoric acid for 15 s can result in mineral dissolution without collagen structure damage. However, longer applications of 37% phosphoric acid within 60 s may increase collagen degradation.