A probabilistic model for estimating the fatigue life of composite laminates based on the mean value and standard deviation of the fatigue life is introduced here for predicting the distribution of fatigue life at any stress level for a constant stress ratio. The proposed model is validated with experimental data for several composite materials, and its accuracy is evaluated by goodness-of-fit-statistical tests. The proposed second-order polynomial model can be used to develop probabilistic constant life diagrams and probabilistic S-N curves for any stress ratio which are corroborated well by available experimental data.