CARBON nanotubes are predicted to have interesting mechanical properties-in particular, high stiffness and axial strength-as a result of their seamless cylindrical graphitic structure(1-5). Their mechanical properties have so far eluded direct measurement, however, because of the van; small dimensions of nanotubes. Here we estimate the Young’s modulus of isolated nanotubes by measuring, in the transmission electron microscope, the amplitude of their intrinsic thermal vibrations, We find that carbon nanotubes have exceptionally high Young’s moduli, in the terapascal (TPa) range. Their high stiffness, coupled with their low density, implies that nanotubes might be useful as nanoscale fibres in strong, lightweight composite materials.