This paper presents a frequency-wavenumber (f-k) domain signal processing approach for guided plate wave data. The guided waves propagate in a steel plate bonded to concrete substrate, and the analysis aims to characterize the steel-concrete interface. The modal solutions of guided waves in well-bonded, partially bonded, and dis-bonded interface cases are obtained using the Global Matrix technique. The analytical solutions demonstrate that the attenuation characteristics of the fundamental symmetric (S0) guided wave mode in the steel plate are sensitive to the steel-concrete interface bond condition. The attenuation behavior of the S0 mode are captured and extracted from the complete guided wave signal set obtained by air-coupled ultrasonic tests. Using f-k domain signal analysis, the fundamental anti-symmetric (A0) mode is suppressed and the S0 mode is isolated. The S0 mode attenuation across the scanned spatial points is then estimated and used to characterize the bond condition of the steel-concrete interface. This signal processing approach is verified by a series of numerical simulation and laboratory-scale experiments. The results demonstrate that interface bond condition can be successfully characterized using the proposed f-k domain signal analysis approach.