The variation of the strength of a grain and a grain boundary was measured by a micro tensile test system fabricated in an electron microscope. A micro-scale beam structure was fabricated from a single-crystalline silicon wafer and a fine nano-scale sample cut from a thin film using a focused ion beam was attached to the center of the beam by deposition of tungsten in the microscope. The other end was attached to a micro probe similarly. Finally, the micro probe was activated to pull the sample and the deformation of the sample was observed by a scanning electron microscope. The fracture strength of a bicrystal sample was measured by detecting the deflection of the beam at the breakage of the sample. The crystallinity of both a grain and a grain boundary was quantitatively evaluated by using electron back-scatter diffraction analysis from the viewpoint of the change of the order of atom arrangement in them. It was found that the strength of a grain and a grain boundary varied clearly depending on their crystallinity. In addition, there was a critical value of the crystallinity at which the fracture mode of a polycrystalline material changed from intergranular fracture to transgranular fracture.