Abstract: The pulse -echo technique was used to obtain the ult rasonic measurement of two different group s ofcarbon fiber reinforced plastic (CFRP) samples , and FFT algorithm was used for f requency spect ral analysis. Theattenuation coefficient shows approximately linear behaviour with f requency over the frequency range used , and theattenuation slope increases with the void content . The attenuation effect s of voids were shown by the dropped amplitude , the downshif ted position and the decreased bandwidth of the center peak. The results indicate that linearrelationships hold between the cent roid f requency shift , the at tenuation slope and the void content , and theserelationships will aid the model development for void content detection using frequency spect ralanalysis.