This paper describes the influences of specimen size, focused ion beam (FIB) induced damage, and high vacuum annealing on the mechanical properties of silicon (Si) nanowires (NWs) evaluated by tensile testing in a FE-SEM. Si NWs made from silicon-on-nothing (SON) membranes are produced by deep reactive ion etching (DRIE) and annealing. FIB system's probe manipulation and film deposition functions are used to directly bond them onto the sample stage of a tensile test device and to fabricate Si NWs. The mean Young's modulus and strength of FIB-damaged NWs are 129.1 GPa and 5.6 GPa, respectively. After annealing, the Young's modulus is increased to 168.4 GPa, whereas the strength is decreased due to morphology degradation.