A theory is developed to predict the evolution of transverse ply cracking in a composite laminate as a function of the underlying statistical fracture toughness and the applied load. Th e instantaneous formation of a matrix crack spanning both the ply thickness and the ply width is assumed to be governed by the energy criterion associated with the material fracture oughness, Γ, at the ply level. Assume multiple matrix fractures occur quasi statically and sequentially such that the ply cracks form one after another under the constant external load imposed on the specimen. Th e number of cracks, n, within the gauge length, 2L, is a discrete random variable for a given applied load, σ, because the fracture toughness varies with the location of fractures in a given specimen as well as from specimen to specimen. Th e probability function f (n, σ, L) of the discrete random variable, n, is determined from the fracture toughness distribution and the solution for the potential energy release rate. Consequently, the distribution of the crack density, dn-n/2L, is obtained. Finally, the mean crack density is formulated as a function of the applied load.