The fiber optic sensor technology has many advantages compared with conventional measuring methods for a smart composite structure. Fiber optic sensors can be used to monitor several parameters during composite cure and to evaluate the structural integrity in service after cure. In this paper, Emphasis was laid upon application methods, basic performances, and practical applications of fiber optic sensors. As the application method of fiber optic sensors, a novel out-of-plane ingress/egress method was proposed using developed fiber protectors, optical part fixtures, and fiber preservation technique. As the basic researches of fiber optic sensors, the strain measurement using an improved AEFPI sensor, simultaneous measurement of strain and temperature using a FBG/EFPI hybrid sensor, and simultaneous measurement of strain, temperature, and vibration using a FBG/EFPI hybrid sensor system were conducted. To improve the performance of AEFPI sensor, the gold was deposited on the reflecting fiber end inside a glass capillary tube. To simultaneously measure the strain and the temperature using a FBG/EFPI sensor, the relationship was analytically derived for the sensor outputs to measurands(strain and temperature) and the characteristic matrix of the sensor was obtained. For the simultaneous measurement of strain, temperature, and vibration, a WSFL system and a laser diode system were combined as the multi-functional fiber optic sensor system by a wavelength division multiplexer. As the practical applications of fiber optic sensors, researches were done for the composite cure monitoring using FBG/EFPI sensors and the strain monitoring of a filament wound composite tank using FBG sensors. Fabrication strains and curing temperatures were monitored simultaneously using embedded FBG/EFPI sensors during cure processes of 4 types of composite laminates such as unidirectional, symmetric cross-ply, unsymmetric cross-ply, and fabric laminates. Also, thermal strains and temperatures of the fabricated unsymmetric laminate were measured afterward in a thermal chamber. 20 FBG sensors were used to monitor the behavior of a filament wound composite tank during a hydrostatic pressurization. The strains measured by FBG sensors were compared with strains measured by strain gages and results of finite element analysis.