Influence of Material and Thickness of the Specimen to Stress Separation of an Infrared Stress Image Kenji MACHIDA The thickness dependency of the temperature image obtained by an infrared thermography was investigated using specimens with three kinds of materials and four kinds of the thickness of the specimen. Only the sum of the principal stresses which is the first invariant of stress tensor is measured, and it is impossible to measure individual stress components directly. Then, the infrared hybrid method was developed to separate individual stress components. Although the form of the contour line of low stress side differs greatly, the distribution form of high stress side was considerably alike. For thin specimens of 1mm thickness, the stress field obtained from the infrared hybrid method differed from it at the free surface which was obtained from 3-D finite element analysis. For specimens thicker than 3 mm, stress intensity factors were nearly uniform irrespective to thermal conductivity, and remarkable thickness dependency of the specimen could not be observed. The stress intensity factor of material with low thermal conductivity can be estimated with high accuracy by the infrared hybrid method. Key words: Infrared Thermography, Thickness Effect, Infrared Hybrid Method, Inverse Problem, Stress Separation, Stress Intensity Factor, Thermal Conduction