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This research explains an experimental investigation of the concrete applying nonlinear ultrasonic testing technique. Eighteen cubic specimens, 150 x 150 x 150 mm, were prepared from three concrete batches with w/c of 40, 50, and 60%, respectively. The hardened concrete specimens after 28-day of curing were simultaneously subjected to uniaxial compression and nonlinear ultrasonic testing. The specimens were damaged by gradually loading them in compression in several steps representing 0, 20, 40, 60, and 80%, respectively, of the specimen's ultimate strength. At the end of each loading step ultrasonic evaluation was performed and the time domain waveforms were recorded at different power levels. Fast Fourier transformation (FFT) of the time domain waveforms was conducted to produce the frequency spectra. The data obtained from frequency spectra was used to study the change in signal amplitude or signal attenuation. It was found that attenuation is quite sensitive to different damage and power levels. The Fourier amplitudes obtained from FFT of the time domain data were normalized at fundamental frequency. Normalized spectra were used to examine the generation of second harmonic wave components. It was also found that the formation of second harmonic components are highly sensitive to increase in damage and change in power levels. DOI: 10.1061/(ASCE)MT.1943-5533.0000050 CE Database subject headings: Concrete; Ultrasonic methods; Damage; Compression; Ultimate strength; Time factors; Transformation; Spectral analysis; Attenuation. Author keywords: Nonlinear ultrasonic; Ultimate strength; Time domain; Fast Fourier; Transformation; Frequency spectra; Normalized spectra; Signal attenuation; Harmonic components.