Residual stresses arise in fiber-reinforced metal matrix composites due to the thermal expansion mismatch between the matrix and fibers after cooling the composites from elevated temperatures. The residual stresses in a 6061Al alloy unidirectionally reinforced with 140-μm diameter SiC fibers were measured during thermal cycling, and after heat-treating, of the composite. While relative changes of the fiber residual stress were estimated from measurements of the change in length of the heat-treated composite, matrix residual stresses were measured by X-ray diffraction. The X-ray triaxial stress analysis, where the measured value of a stress-free interplanar spacing d0 was discussed to be reliable, showed that a stress state in the matrix surface layer sampled by the X-ray was biaxial and that the longitudinal residual stress parallel to the fibers was the maximum principal stress. It was found that the residual stresses were independent of cooling rates of the composite and that changes of the longitudinal residual stress in the matrix and in the fibers balanced each other in the heat-treated composite. The X-ray biaxial stress measurements during thermal cycling between room and aging temperature of the aged composite revealed that the matrix tensile residual stresses decreased linearly with increasing temperature. The reduction could be well described by using an elastic concentric cylinder model.