Fiber-reinforced polymer (FRP) composites are known for their outstanding strength, stiffness, corrosion resistance, and light weight nature, which are used in such fields as aerospace, marine, automotive, and construction industries. Although, FRP composites have many advantages, as compared to metals, defects may occur during the production process or during the service life due of the laminated structure. Therefore, there is a growing demand for nondestructive testing (NDT) approaches that can pre-detect such defects before they become critical and cause failure. Radio frequency (RF) signals are able to penetrate dielectric materials with relatively low loss, which make microwave NDT an attractive solution among the other numerous inspection techniques. This paper presents the theory and development of an ultrawideband (UWB) microwave imaging system, covering 8.2 to 12.4 GHz frequency band, based synthetic aperture radar (SAR) algorithms designed to detect damages and defects within the FRP structures.