Magnesium is a light metal, with a density two-thirds that of aluminium, is abundant on Earth and is biocompatible; it thus has the potential to improve energy efficiency and system performance in aerospace, automobile, defence, mobile electronics and biomedical applications1,2,3,4,5. However, conventional synthesis and processing methods (alloying and thermomechanical processing) have reached certain limits in further improving the properties of magnesium and other metals6. Ceramic particles have been introduced into metal matrices to improve the strength of the metals7, but unfortunately, ceramic microparticles severely degrade the plasticity and machinability of metals7, and nanoparticles, although they have the potential to improve strength while maintaining or even improving the plasticity of metals8,9, are difficult to disperse uniformly in metal matrices10,11,12,13,14. Here we show that a dense uniform dispersion of silicon carbide nanoparticles (14 per cent by volume) in magnesium can be achieved through a nanoparticle self-stabilization mechanism in molten metal. An enhancement of strength, stiffness, plasticity and high-temperature stability is simultaneously achieved, delivering a higher specific yield strength and higher specific modulus than almost all structural metals.