When the weak interface of composite materials subjected to the transverse tensile, composites with weak interface involve interfacial cracks before the overall damage of the material. The measured transverse tensile strength of the weak interfacial bonding composite is even much smaller than the original tensile strength of the pure matrix. The stress concentration factor (SCF) is defined considering the existence of cracks on interfaces with in-situ strength of the matrix assigned as its original counterpart divided by the SCF. A new method is provided to predict the load level when the interface crack occurs and the strengths of composites under uniaxial tensile.