Thermal stress behavior of W/Cu composites was studied by the X-ray stress measurement method.With the X-ray technique, the thermal stress of the composites due to the thermal expansion coefficient mismatch of component materials has been measured in the form of residual stress at room temperature. This paper reports the thermal stress behavior during heating and cooling of the composites, which was measured by using a new apparatus consisting of an X-ray diffractometer and a vacuum furnace.Generally, the thermal stress behavior of composites is given by the following equation.