Thermal stress relaxtion at the interfaces in magnesium matrix composite has been studied by mechanical spectroscopy. Two specimens were processed by a gas pressure infiltration method. In one sample, the metallic matrix was unidirectionally oriented with respect to the matrix-reeinforcement interface by a Bridgeman technique, while the other was kept as infiltrated. The results show a strong dependence of the damping on the orientation of the matrix. The results are compared with a model of thermal stress relaxation by dislocation motion controlled by a solid friction mechanism. A fitting parameter proportional to the mobile dislocation density is obtained showing the entire evolution of the mobile dislocation density with respect to the temperature.