A non-destructive testing method using ultrasonic waves for composite materials has been studied. In this paper, a computer program of three dimensional image processing was developed to evaluate the defects in composite materials. The developed computer program has two particular functions, which are smoothing of scan data and decision of defect size. The size of defect was obtained by a gradient of echo level. As the examples, this method was applied to GFRP and CFRP laminates with artificial defects, and the effect of processing on the size of defects was investigated. As a result, the shape, size and location of defects in composite materials were obtained by the three dimensional image processing. It is also revealed that the detectability of defects is improved by this method. From these results, it can be shown that the method proposed in this paper is very useful to evaluate defects in composite materials.