Single collagen fibers and nanohydroxyapatite/collagen nanocomposite fibers with 6.4-18.5 wt% hydroxyapatite (HA) content were prepared by electrospinning. Their mechanical properties were systematically investigated at ambient conditions by means of nanotensile tests. A narrow range of fiber diameters, 250-350 nm, was selected for these tests as size effects are observed for the mechanical properties of all the fibers types, namely a decrease as the fiber diameter increases. The pure collagen fibers are found to exhibit tensile properties comparable to natural collagen fibril. Young's modulus of the HA-filled nanocomposite fibers is found to be only slightly higher than that of the pure collagen fibers, but significant improvements in strength, strain and toughness, are obtained. Optimal mechanical properties wise in the 6.4-11.6 wt% HA range. (C) 2012 Elsevier Ltd. All rights reserved.