In this dissertation, I have investigated and developed a thorough understanding of the mechanical properties of diamond-structured semiconductor materials at small length scales over a broad temperature range. These materials are routinely used in a wide variety of microelectromechanical systems (MEMS), as well as functional semiconductor components in micro-devices. Previous observations of plasticity in these apparently brittle materials have only been achieved by macro-scale testing at high temperatures above brittle-to-ductile transition. This left a large gap in the knowledge of the plasticity and strength of semiconductors, especially at relatively low temperatures. This resulted from plasticity being prevented by premature brittle fracture of strongly covalent diamond-structured crystals. Recent advances in micromechanical testing techniques have revealed that materials exhibit significantly enhanced strength and plasticity at small length scales, from microns to submicrons, in contrast to conventional bulk behavior. This created a pathway for understanding plasticity in apparently brittle semiconductors by exploiting and studying size effects on mechanical properties and deformation behavior. This thesis focuses on the study of the mechanical properties of the three diamondstructured Group-IV semiconductors – diamond, silicon and germanium – at small scales over a wide temperature range. In the present study, catastrophic failure was effectively suppressed by performing compression tests of micro-sized pillars with defect-scarce internal microstructures. This approach enables the study of temperature-dependent plasticity and sizedependent strength of these brittle semiconductors. The thesis firstly presents an overview of deformation mechanisms and corresponding dislocation transitions in diamond-structured crystals from cryogenic to high temperatures (0.07−0.56 Tm). Secondly, size effects are investigated in these covalent semiconductors which possess high Peierls’ stresses in comparison with metallic and ionic crystals investigated in the literature. Here, diamond, Si and Ge display a weaker size effect than other crystals with metallic and ionic bonds. The size effect is seen to significantly decrease with increasing normalized shear stress (τ/G) in crystals with various chemical bonds. As the temperature increases and Peierls’ stress decreases, all materials displayed increasing size effects. In addition, extraordinary properties and interesting behaviors were observed in these materials at small length scales. These are as follows: (1) the first observation of notable plasticity in diamond at low temperature 400 °C (0.16 Tm), which significantly extends its plastic regime below 1000 °C; (2) irradiation damage of Helium ions on the structure and strength of diamond was found to be highly orientation-dependent with 〈100〉-oriented diamonds exhibiting superior damage resistance due to higher ion channeling efficiency; (3) Si processed by modern lithography procedure and surface cleaning exhibits an ultrahigh elastic iv strain limit, near ideal strength (shear strength ~4 GPa) and plastic deformation at the micronscale, one magnitude larger than previous observations, due to superior surface quality; (4) a transition in deformation mechanisms from full to partial dislocations was observed by increasing specimen size in Si at ambient temperature, indicating the defect transition is also stress-dependent in addition to temperature-dependent.