A model is developed to analyze fiber/matrix debond growth along the broken fiber interface in a single fiber composite subjected to tension-tension fatigue. Paris law expressed in terms of debond length growth rate and strain energy release rate is used. Analytical solution for Mode II energy release rate GII is obtained for long debonds where the interface crack growth is self-similar. For short debonds the interface crack interacts with the fiber break and therefore FEM modeling in combination with the virtual crack closure technique was performed to calculate the increase of GII. Finally, the calculated GII dependences are summarized in simple expressions that are used to simulate debond growth in fatigue. A parametric study of the effect of Paris law parameters on debond growth is performed.