The aim of this work was to correlate the plastic pre-straining with the relaxation of the residual stress, RS, of several metal matrix composites that underwent different levels of plastic deformation. RS were calculated from neutron diffraction measurements. Results show that both macro and micro-stresses decrease rapidly with only 1% plastic strain; little relaxation occurs with further strain. The hydrostatic component of the micro-stress relaxes, while the deviatoric one stays constant.