AbstractAn experimental study of fatigue crack growth in dentin was conducted, and the influence of stress ratio (R) on the crack growth rate and mechanisms of cyclic extension were examined. Double Cantilever Beam (DCB) fatigue specimens were sectioned from bovine molars and then subjected to high cycle fatigue loading (105 < N < 106) under hydrated conditions. The evaluation consisted of Mode I loads with stress ratios that ranged from −0.5 to 0.5. The fatigue crack growth rates were measured and used to estimate the crack growth exponent (m) and coefficient (C) according to the Paris Law model. The fatigue crack growth rates for steady‐state extension (Region II) ranged from 1E‐7 to 1E‐4 mm/cycle. It was found that the rate of cyclic extension increased significantly with increasing R, and that the highest average crack growth rate occurred at a stress ratio of 0.5. However, the crack growth exponent decreased with increasing R from an average of 4.6 (R = 0.10) to 2.7 (R = 0.50). The stress intensity threshold for crack growth decreased with increasing R as well. Results from this study suggest that an increase in the cyclic stress ratio facilitates fatigue crack growth in dentin and increases the rate of cyclic extension, both of which are critical concerns in minimizing tooth fractures and maintaining lifelong oral health. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res 73A: 201–212, 2005