Abstract.The random critical-core model is adapted to investigate the tensile failure mechanism and hybrid effect of unidirectionally arrayed hybrid composites with alternating low and high elongation fibers. By utilizing the model in conjunction with the results of the stress concentration analysis in which the interfacial damage between fiber and matrix is considered, a microscopic statistical analysis of both the first failure and ultimate failure of hybrids is performed. The variations of the first failure strain, the ultimate failure strain and the hybrid effect as the interfacial shear strength are obtained quantitatively. The concept of the hybrid effect for strains has been clarified. The present results are compared with available experiment data and a reasonable agreement is found between the analytical predictions and the experimental results.