Silicon is widely used within energy, electro-mechanical, environmental devices by nanostructural control. As silicon parts constitute structural components whose size is ever decreasing, it is critical to understand the mechanical properties of single crystal silicon from precise measurements of load and displacement using microscopic sample in sub-micron and macroscopic scales. Here, the mechanical properties of single crystal silicon were precisely evaluated by bending tests at room temperature using microcantilever beam specimens having a several micron size. The microcantilever beam specimens were prepared using a focused ion beam technique, followed by loading the tip of the specimens. The smaller specimens deformed nonlinearly and then fractured. The unloaded specimen after nonlinear deformation showed permanent strain and many dislocations close to the region where high tensile stress was applied. This means that the nonlinear stress-strain relationship in the very high bending stress is determined by plastic deformation controlled by dislocation despite occurring at room temperature. The bending strength increased with a decrease in specimen size, and the smallest specimens had close to ideal strength. The size of the region where the dislocations accumulated in high density corresponded to the flaw size estimated from the fracture mechanics. This means that the bending strength of the microcantilever beam specimens of silicon is dominated by newly generated defects resulting from dislocations; in other words, the size effect of bending strength of silicon at the micrometer scale is controlled by the accumulation of newly formed dislocations because the dense dislocation region should be lower in a smaller-sized specimen.