This　paper　provides　a　comprehensive　review　on　the　methodological　development　and　technical　applications　of　in　situ　microscopy,　including　transmission　electron　microscopy　(TEM),　scanning　electron　microscopy　(SEM)　and　atomic　force　microscopy　(AFM),　developed　in　the　last　decade　for　investigating　the　structure-mechanical-property　relationship　of　a　single　one-dimensional　nanomaterial,　such　as　nanotube,　nanowire　and　nanobelt.　The　paper　covers　both　the　fundamental　methods　and　detailed　applications,　including　AFM-based　static　elastic　and　plastic　measurements　of　a　carbon　nanotube,　external　field-induced　resonance　dynamic　measurement　of　elastic　modulus　of　a　nanotube/nanowire,　nano-indentation,　and　in　situ　plastic　deformation　process　of　a　nanowire.　Details　are　presented　on　the　elastic　property　measurements　and　direct　imaging　of　plastic　to　superplastic　behavior　of　semiconductor　nanowires　at　atomic　resolution,　providing　quantitative　information　on　the　mechanical　behavior　of　nanomaterials.　The　studies　on　the　Si　and　SiC　nanowires　clearly　demonstrated　their　distinct,　＂unexpected＂　and　superior　plastic　mechanical　properties.　Finally,　a　perspective　is　given　on　the　future　of　nanomechanics.