Vapor-grown carbon nanofiber (CNF)/epoxy composites are characterized under compression at 5 9 10(-3)-2800 s(-1) strain rates. A difference in the fiber failure mechanism is identified based on the strain rate. CNFs show signs of deformation along their entire length under quasi-static compression. In contrast, the high-strain rate failure results in rupture of outer turbostratic carbon layers, leading to stress transfer to the inner graphite layers. The graphitic layers elongate and rupture, forming a conical tip at the fracture cross section of the CNFs. The strength of nanocomposites at high strain rate is measured to be up to 180 % higher depending on the composite composition and strain rate. CNFs substantially increase the localized plastic deformation of the matrix under quasi-static compression and result in nanoscale deformation features on the failure surface. The observed higher strength and modulus of nanocomposites at high strain rates are attributed to the difference in the matrix and fiber failure mechanisms at different strain rates.