Thermographic non-destructive testing (NDT) based on the thermal resistance effect of defects is developed for the inspection of delaminated and sandwiched defects embedded in composite structures. The resolution is examined for the artificial delaminated defects in carbon fiber honeycomb structures using the conventional infrared radiation heating. Experimental results demonstrate that the radiation heating is effective for the accurate defect inspection of composite structure samples. We have developed an experimental and computational hybrid system which can be applied to detect defects in various composite structures. The system consists of an infrared thermal video system by which the temperature distribution of body surface can be measured, a computer with a PIP-1024B image board carrying out image processing of thermograms, and a HP ink jet XL printer. By applying the system to honeycomb sandwich structures and delaminated composite structures, various types of defects lying between the honeycomb core and the surfaces as well as between the internal layer and the surface of delaminated structures have been examined. It is found that the method is extremely applicable to defect detection of honeycomb sandwich structures and composite materials delaminated structures.