Flexible Rutherford cables are needed to realize high field superconducting magnets with A15 conductors based on the react-and-wind (R&W) technology. Aiming such an application, ultra-thin A15 composite wires with a diameter of 0.03-0.05 mm have been developed by the National Institute for Materials Science (NIMS). Mechanical properties of such ultra-thin Nb 3 Sn wires were evaluated to determine the cabling parameters and mechanical analysis of twisted cables. Tensile tests were performed at room temperature for 0.05 mm-thick Nb 3 Sn wires before and after heat treatment for the first time. Basic mechanical parameters such as 0.2% proof strength and fracture strength were evaluated from a stress-strain curve. Young's modulus of such a thin wire was determined from unlading and reloading slopes of a load-stroke curve for the specimens with different gauge lengths. Fracture strain was estimated without using extensometers and strain gauges by correcting for machine deformation. Based on these results, we concluded that a simple technique to measure stress-strain curves for ultra-thin Nb 3 Sn wires was able to be established.