Abstract: Interlayer(or interphase) fracture constitutes an important issue of the micromechanics of composite materials.The present paper studies the fracture behavior of a crack penetrating vertically to an interlayer where the stiffness varies monotonically from one material phase to another material phase.Analytical estimate and full-scaled FEM calculation both indicate that the crack-tip stress intensity factor increases when a crack penetrates the interlayer from the soft material side,and decreases when a crack penetratesthe interlayer from the stiff material side of the bimaterial.