Metal Matrix Composites (MMCs) are potential candidate materials in the aerospace and automobile industries because of its attractive properties, in particular, their high specific properties, and Superplastic forming (SPF) is a good solution to the problems in the forming process of MMCs due to their low ductility resulting from the incorporation of reinforcement. High strain rate superplasticity (HSRS) is attractive for industrial applications because superplastic forming at high strain rates can reduce forming time greatly. The strength of P/M 6061 Al and 6061 Al/SiCp (3 mum) composites during superplastic deformation at temperatures of 853 K-871 K and a high strain rate of 0.1 s(-1) has been studied in this paper. Experimental results presented a softening effect by the SiCp reinforcement. Mechanical and microstructural analyses show that the decrease in the strength during high strain rate superlastic (HSRS) deformation is associated with the decreased grain size of the Al matrix with increase of the SiCp volume fraction or the extrusion ratio, and the occurrence of liquid phase. The formation of the liquid phase was related to segregation of the solute atom during HSRS deformation. (C) 2003 Kluwer Academic Publishers.