Interfacial shear strength (IFSS) between fiber and matrix is one of the most important factors in characterizing the mechanical properties of fiber reinforced composites. If the IFSS is too low, it’s hard to expect the performance of reinforcing fibers in composites, whereas if the IFSS is too high, there is a decrease in fracture toughness of composites because of the poor resistance to the stress crack propagation. Hence, it is necessary that the IFSS should be determined via the optimization rather than the maximization for the purpose. Several micro-mechanical techniques were proposed for measuring IFSS in composites. Some of the most frequently used techniques include the single fiber pull-out test[1], the single fiber composite (SFC) test[2], and micro-indentation method[3]. Among them, the SFC test, originally proposed by Kelly and Tyson[4], has received a lot of attention both as a diagnostic for fiber/matrix adhesion and as a simple composite system composed of the elastic fiber imbedded in a plastic matrix, Kelly and Tyson showed that the critical fragment length lc is given by lc=dσf2τi where d is the fiber diameter, τi is the IFSS, σf is the fiber fracture stress.