Nano/micron-size fibers are of growing interest not only in the textile industry but also in non-textile industries because of their superior functional properties. Since a commercial machine has not been designed to measure the mechanical properties of nano/micron-size fibers in any environment, we developed a versatile tensile tester suitable for monofilaments of nanometer to micrometer size. The design of the tester provides the following advantages : 1) convenient and precise mounting of fiber in a fiber holding unit 2) test methods conducted not only in ambient air but also in an ambient liquid environment, and 3) multiple tests including not only the simple tensile test but also the stress relaxation test. and three-point flexural test.Using the developed tester, the mechanical properties of micron-size polyethylene, glass and carbon fibers as well as electrospun nylon-6 nanofiber are well evaluated with a proposed procedure. The characteristic feature of Young' s modulus, the breaking stress, the stress relaxation of the single micro-and nanofibers are studied in terms of the fiber diameter, the gauge length, and the environment (air or water).