Tensile properties of collagen fibers of approximately 1 μm in diameter were determined using a newly developed micro tensile test system for cells and fine fibrous biological tissues. The test system consists of a thermostatic test chamber, an inverted microscope, micromanipulators, a direct drive linear actuator, a cantilever-type load cell, and a video dimension analyzer (VDA). The fibers were isolated with a mechanical method from collagen fascicles (approximately 300 μm in diameter) cut out from the rabbit patellar tendon. The ends of each fiber were attached to the tips of a pair of glass microtubes (15 to 20 μm in outer diameter) using a cyanoacrylate adhesive. One of the microtubes was attached to the load cell;​ the other one was connected to the linear actuator which was utilized to stretch the fiber. Load applied to the fiber was measured with the load cell, while its elongation was determined with the VDA using the images of the edges of the adhesive as markers. Tangent modulus, tensile strength, and strain at failure of the tested fibers were 54.3±25.1 MPa, 8.5±2.6 MPa, and 21.6±3.0%, respectively. These values were much different from those of collagen fascicles (300 mm in diameter) cut out from the rabbit patellar tendon and also from those of the bulk patellar tendon (Trans. ASME, J. Biomech. Eng. 121, 124–294, 1999);​ for example, tensile strength and strain at failure of the fibers were approximately 50 and 200% of those of the fascicles, respectively. These results suggest that the mechanical interactions between fibers and between fibers and ground substances contribute much to the mechanical properties of collagen fascicles and bulk tendons.