Microwave and ultrasonic methods have been used for the detection of debonding between concrete and fiber-reinforced plastic (FRP). To determine the capability of the microwave method in detecting thin delamination between two materials of concrete and FRP, concrete specimens are made with the dimensions of 600 mm (length) x 600 mm (width) x 50 mm (depth). Specimens have artificial delamination of 5 and 10 mm thick Styrofoam, which represent debonding in structures. Then, the specimens are partially covered with 1.5 mm thick FRP on the top of 3 mm thick epoxy. A horn antenna with a center frequency of 15 GHz and a frequency bandwidth of 10 GHz is used for the measurements. By transmitting and receiving microwave signals from the horn antenna, differences have been detected for the different types of the targets. Also, measurements using ultrasonic method at 5 kHz have been made on the same series of specimens. This work is directed toward a development of an effective and practical microwave based non-destructive evaluation methodology for the detection and quantification of damages in FRP-covered reinforced concrete members in bridges and buildings.