A better understanding of the debond and pull-out processes is needed because they affect the fracture toughness of fiber reinforced composites. In this paper, a new stochastic model is proposed to predict the probability distributions of debond and pull-out lengths of fibers in unidirectional fiber reinforced composities.The stress distribution in a debonded fiber was determined under existence of friction between the fiber and matrix. By using this stress distribution and a weakest-link model, the probability distributions of debond and pull-out lengths were predicted. When the debond stress is assumed to be scatterd, the computational results predicted by the new stochastic model have good agreement with the experimental data by Beaumont et al.