Objective: The purpose of this study was to characterize collagen cross-linking in carious dentin layers of human permanent teeth. Materials and Methods: Twelve carious teeth were obtained from 9-59 year-old patients and three layers of dentin, i.e. outer, inner carious dentin and normal dentin, were dissected from each tooth. The samples were pulverized, demineralized, reduced with standardized NaB3H4, hydrolyzed with 6N HCl and subjected to quantitative collagen cross-link analyses. The amounts of cross-links were calculated on moles per mole of collagen basis and the values were compared using ANOVA and LSD with the significance level of 0.05. Result: The study showed that reducible cross-links (dehydro-dihydroxylysinorleucine, dehydro-hydroxylysinorleucine) and non-reducible cross-links (pyridinoline, deoxypyridinoline) were significantly different among the layers examined (p<.05). All of these cross-links in inner carious dentin were higher than those of outer and normal dentin. In the outer carious dentin, only dehydro-dihydroxylysinorleucine was lower when compared to that of the other two. Conclusion: The data indicate that the dentinal caries process may alter collagen cross-linking pattern and that though in the severely infected carious region reducible cross-links are degraded, the nonreducible pyridinium cross-links are still structurally stable. This study was supported by Faculty of Dentistry, Mahidol University and NIH-NIDCR DE 10489.