In previous research, the modified equivalent inclusion method developed by Taya and Chou was extended to be applicable to the case where there were many kinds of inhomogenities and inhomobeneous inclusions. By using this extended method, micromechanical analysis was performed on a discontinuous fiber-reinforced composite showing interfacial sliding between fiber and matrix, and analytical experssions such as for energy release rates and the overall elastic modulus as a composite were derived. In addition, the fracture toughness of such a composite was evaluated in terms of the critical size of a crack which travels across the composite in an unstable manner. The effect of material parameters on the toughness can be explained by the interfacial phenomenon, whose results are consistent with common experimental results. In the present paper, by applying the extended modified equivalent inclusion method, micromechanical analysis is done of the unidirectional fiber-reinforced composites producing interfacial debonding between fiber and matrix. The expressions of energy release rates both for a matrix crack and a debonding one can be obtained.