Abstract: In order to provide the theory and operation parameter windows for the modulated thermal wave imaging of composite materials, the rules of modulated thermography for delamination in composite materials were studied using computer simulation based on the finite element method. The relations of phase difference versus modulation frequency and defect depth were analyzed. The prediction of the best modulation frequency, half-amplitude frequency band and blind frequency were presented. The evaluation method of the defect depth by using the blind frequency was verified, and a new approach of defect depth evaluation by using the best modulation frequency was presented. The results show that both of the best modulation frequency and the blind frequency are approximately an inverse proportion function of the defect depth squared. The maximum phase difference and the width of the half-amplitude frequency band decrease when the defect depth increases. The defect depth can be estimated from the blind frequency or the best modulation frequency, and the estimated errors are about 9% and 11% respectively within the defect depth range of 1-4 mm.