One dimensional (1-D) metallic nanostructures (e.g. nanowires, nanorods) have stimulated great interest recently as important building blocks for future nanoscale electronic and electromechanical devices. In this thesis work, gold and nickel nanowires with various diameters were successfully fabricated, and two dedicated platforms, based on (1) a novel micro mechanical device (MMD) assisted with a quantitative nanoindenter and (2) a TEM-AFM sample holder system, were developed and adopted to perform in situ tensile tests inside SEM and TEM on samples with diameter ranging from a few nanometers to hundreds nanometers. Size-dependent mechanical behavior and different fracture mechanisms of gold nanowires had been revealed and discussed. In addition, we discovered cold welding phenomenon for ultrathin gold nanowires (diameter < 10nm), which is anticipated to have potential applications in the future bottom-up integration of metallic 1-D nanostructures and next-generation interconnects for extremely dense logic circuits.