This paper presents the current status of the development of nanostructured refractory transition metals of W and Mo, with emphasis on the effects of nano-structures upon the mitigation of embrittlement by high ductile-to-brittle transition temperature (low temperature embrittlement) and by irradiation (radiation embrittlement) that is the key issue of the materials for use in extreme environments. An overview of processing of nano-structured refractory metals with and without dispersoids is given. The features of microstructures of the refractory metals are described in connection with grain growth behavior due to very high internal energy of nano-structures. Effects of nanostructures on physical and mechanical properties and radiation performance with neutron and light ions such as 3MeV He 4+ and 1keV H 3+ are presented. It is shown that grain boundaries are not only effective sinks for irradiation-induced point defects, but also are distributors of implanted light ions by enhanced diffusion in nano-structured refractory metals.