The fatigue crack growth rate in strain crystallizing rubbers depends not only on the maximum energy release rate experienced during a loading cycle, but also on the minimum, or equivalently, the R ratio. The R ratio is the quotient of the minimum and maximum energy release rates occurring during a loading cycle. This article proposes a simple phenomenological model for the effect of R ratio on fatigue crack growth rate in strain crystallizing rubbers. The model may also be applied for Wohler-style (S-N style) fatigue life curves. The model is compared against experimental results reported by Lindley, by Cadwell, Merrill, Sloman, and Yost, and by the authors. The ability of the model to represent the experimental data is found to be reasonable. The model provides a simple approach to estimating fatigue performance when limited data are available, and may find application in the analysis of load histories from actual service. Such variable amplitude load histories typically contain cycles covering a wide range of R ratios.