Abstract: Void content non-destructive evaluation oriented, an ultrasonic attenuation model of carbon fibre reinforced plastics with voids is presented. A relationship formula between the ultrasonic at tenuation, the main parameters of the material and the voids contained is also given. The model and the formula give support to a new method for non-dest uctive evaluation of void content of carbon-fibre reinforced plastics in our next paper. As a partial result, a sub-model of ultrasonic attenuation of fibre within plastics is established. The relationship between the ultrasonic at tenuation and content and the radius of fibre is accordingly derived out and presented. Another partial result gives a sub-madel of at tenuation due to the socalled distributed voids, in which it is considered that the content of voids is relative to their sizes (e. g. radius of voids) and the property is different in different void sizes. So the statistics property of the distribution of the sizes of the voids is int roduced into the single radius based model.