Abstract: A new method is proposed for non-destructive evaluation (NDE) of void content of carbon-fibre reinforced plastics. The method establishes a testing model which is based on a theoretical model and calibrated by experiment. Thus the problem of the disagreement between present models and experiment results is well solved. Under reasonable simplifying conditions, the detection relationship between ultrasonic attenuation and void content established is simple for practice. The unif ying form of the relationship has general meanings and the application formula calibrated by experiment is just fit to given materials. Attention is paid in this paper both to the conformability and reliability of the method.