The author and Hitachi Cable, Ltd. have recently developed small-diameter fiber Bragg grating (FBG) sensors in order to embed the sensors into composite laminates without strength reduction. This paper presents applications of the small-diameter FBG sensors for the detection of microscopic cracks and delamination in carbon fiber reinforced plastic (CFRP) laminates using the changes in the form of the reflection spectrum. The FBG sensors were also successfully used to monitor the thermal residual stress during the fabrication process. Furthermore, applications of small-diameter optical fiber sensors were demonstrated in a stiffened CFRP fuselage structure of 1.5m in diameter and 3m in length. The optical fiber sensors were successfully embedded into the upper panel of the fuselage, and the real-time detection system could well detect impact locations and impact-induced damages.