Prevention of serious damage of marine structures might be achieved by monitoring the loading conditions and by inspecting the structural integrity. The concept of smart structures with a fiber-optic sensor network can be applied to marine applications. There are plenty of marine structures in huge dimensions, for example, the overall length of a very large crude carrier exceeds 200 m. A fiber-optic laser-Doppler velocimeter was developed as a displacement sensor with long gage length. The optical fiber sensor has potential advantages as a structure monitoring sensor: (i) unlimited gage length, (ii) applicability to dynamic measurement and (iii) less affected by temperature. Free vibration behavior of the coupon specimen could be monitored with excellent dynamic resolution. Flexural deformation of the specimen under cyclic bending load was also monitored, and linearity and sensitivity of the developed sensor system was confirmed. An optical time-domain reflectometer (OTDR) was applied to the damage detection of composite materials in which optical fibers were embedded. Onset and location of fiber breaks could be detected by using the OTDR technique, which was applied to tensile tests of resin, and tensile and flexure tests of cross-plied GFRP. The failure strain of the optical fiber had rather wide scatter when it was embedded in the composite, and the average values also depended on the condition of embedment. Location of the failure points could be measured with sufficient accuracy.