The basic technologies of the impact damage detection system (IDDS) of composite structures were developed and demonstrated using a composite structure with embedded small-diameter optical fiber sensors in FY2002. In our current R&D, the IDDS consisting of optical fiber sensors installed in a composite structure and an interrogation unit is developed for practical airframe application. Under our R&D plan to proceed towards product, cyclic loading tests using different types of composite specimens are conducted to verify the effect of IDDS. In that test using coupon specimens, how much those optical fibers are involved in the mechanical properties of the composites, and the effect of optical signals are investigated. IDDS capability is also evaluated by an impact loading test using a stiffened panel subjected to a cyclic loading.