Abstract: A model of energy release rate for fracture in unidirectional fiber-reinforced composite materials is presented. From the observations in experiments, a fracture criterion for crack initiation in off-axis unidirectional composite materials is proposed. It includes two assumptions: （ 1 ） Crack initiation takes place in the direction of fibers; （2） Rapid crack growth occurs when the energy release rate reaches a critical value, G= G. The model is also used to analyze the mixed-mode fracture of a kind of glass-epoxy unidirectional composite with crack. The results indicate that the loads corresponding to fracture predicted by the model when crack orients at some angle with respect to the fiber direction agree well with the experimental results.