An embedded fiber Bragg grating sensor-based wing load monitoring system for composite aircraft was presented. The presented system was implemented to an actual composite aircraft, and the implemented system was verified through flight tests. During the manufacturing process of the aircraft, fiber Bragg grating sensors were embedded into the composite wing, and the interrogation and data processing system for in-flight measurement of strain distribution and flight parameters and estimation of wing load was installed on-board the aircraft fuselage. Through load calibration tests, wing loads were estimated using strain responses from the embedded fiber Bragg grating sensors, and in-flight wing load estimation was performed. Through flight tests, flight data were successfully acquired, and the acquired flight data were comprehensively analyzed to verify the performance and effectiveness of the implemented system. As a result, the total wing load of the test-bed aircraft was estimated with an average error of 4.19%, and the wing load history reasonably corresponded to the loading conditions for various flight states, such as take-off, climb and descent, and banked turn.