Abstract: This paper proposes a microscopic statistical theory for the ultimate tensile failure of unidirectionally interlaminated hybrid composites with interply interface failure, based on the random critical core theory in conjunction with the results of the stress concentration which are obtained by a shear lag model. A function of the strength distribution and a failure criterion with the effect of interfacial shear strength are derived and a relationship between the ultimate tensile strength and interfacial shear strength is obtained. The results show that the ultimate strength of hybrid composites, interfacial shear strength and hybrid ratio have an optimal match relation, and that the present failure strains are relatively close to the existing experimental value.