An individual carbon nanocoil was clamped between two AFM cantilevers and loaded in tension to a maximum relative elongation of 42%. The deformation of the nanocoil agrees well with an analytical model of the spring constant that accounts for the geometric nonlinearity. The nanocoil behaves like an elastic spring with a spring constant K of 0.12 N/m in the low strain region. No plastic deformation was detected. High-resolution microscopy images and the electron energy loss spectrum (EELS) indicate that the nanocoils are amorphous with a sp 2 /sp 3 bonded-carbon ratio of 4:1. Carbon micro- and nanocoils have been synthesized and studied. 1-6 Because of their unique 3D structure, they have potential applications as mechanical components such as resonating elements and nanosprings or as a novel reinforcement in high-strain composites. Other potential applications are as nanosolenoids and electromagnetic wave absorbers. Volodin et al. 7 reported the elastic properties of coiled carbon nanotubes as measured with force modulation microscopy (an AFM technique). The tensile properties of helical diamond microfibers several millimeters in length and having a wire core of platinum have been discussed. 8 Nakayama et al. 9 presented a brief report on the mechanical properties of carbon nanocoils, and Motojima et al. 10 described the mechanical response of carbon microcoils under extension. Amorphous helical SiO2 nanosprings have also been recently characterized by scanning and transmission electron microscopy and atomic force microscopy 11 . However, a direct experimental measurement of the mechanical response of carbon nanocoils under tensile loading has not yet been conducted and is the focus of the present study. This carbon nanocoil was studied using a home-built nanomanipulator tool operated inside of a scanning electron microscope (SEM, LEO 1525). The nanocoil was picked up and then clamped between two AFM cantilever tips with the electron beam-induced deposition (EBID) method. 12,13 A loading experiment was conducted in which the nanocoil was monotonically loaded/unloaded in tension to a maximum coil extension of 33%. From the load versus elongation data, we have fit the spring constant (K) values for the nanocoil. (k is used later for the spring constant of the AFM cantilevers.) We derived an equation that expresses K in terms of the coil geometry and the shear modulus G and have used this equation to fit G from the measured values of K and the geometric parameters. In addition to our mechanical measurements, we present results of the structural analysis of other carbon nanocoils from the same sample, as obtained by transmission electron microscopy (TEM, Hitachi HF-2000 and Hitachi H-8100).