Abstract: The flow stresses of several aluminium alloys and SiC_W reinforced composites during tensile deformation have been studied. It shows that the strengthening effect of the composites relates to the strength of the matrix alloys; the rule of mixture (ROM) is not applicable for the prediction of the tensile strength of these composites; the yield stresses of those composites with higher strength matrix alloys are lower than those of the matrix alloys, and the work hardening ability of the composites is much higher than that of the matrix alloys. Some possible reasons of these results are discussed in this paper.