Classical fracture mechanics and conventional dislocation based understanding of metal yield and failure was well researched and understood until the recent advancement of electron microscopy techniques and the ability to produce small-scale metal samples. Once a critical dimension with respect to grain or geometrical size for a metal was reached, the deformation and fracture behaviors began to deviate from traditional theories and predictions. Recent research has focused on one-dimensional (1D) nanomaterials produced from their bulk counterparts to probe corresponding mechanical behaviors. However, this could alter the intrinsic material properties due to modifications caused by the milling/machining process. In this thesis, metal nanowires/fibers produced by an alternative process, and samples with controlled defect density, were used to gain a fundamental understanding of the critical dimension effects and small-scale mechanical behavior of metals. Another important aspect of this thesis is the development of a state-of-the-art nanomechanical characterization technique in the transmission electron microscope, where a detailed structure-property relationship could be reliably established with unprecedented resolutions. In the first part, single crystalline silver fibers were grown with dimensions varying from 2μm down to 400 nm and investigated using an advanced in-situ method. A critical dimension was observed near 1μm, below which the yield stress began to increase and the number of deformation slip bands began to decrease. Secondly, single crystal Mo-alloy fibers produced by a similar technique were pre-strained to introduce pre-existing dislocations and simulate bulk characteristics. In-situ tensile test showed the pre-straining effect on the yield stress, and the existence of a negative strain rate sensitivity. Next, pristine Mo-alloy fibers were irradiated with He and Ar ions to test the effects of irradiation induced defects on their mechanical behaviors at this length scale. A clear size effect was observed depending on the damage produced by the irradiation. Finally, a detailed description of an in-situ quantitative TEM stage based on a MEMS device and its successful applications are presented. This set-up was designed and built to compliment the in-situ SEM test by offering high resolution observations of the microstructural evolution under testing conditions and providing additional insights to the important structure-property relationship.