A thermo-morphic transition of a silicon nanowire (Si-NW) is investigated in vacuum and air ambients, and notable differences are found under each ambient. In the vacuum ambient, permanent electrical breakdown occurs as a result of the Joule self-heating arising from the applied voltage across both ends of the Si-NW. The resulting current abruptly declines from a maximum value at the breakdown voltage (V-BD) to zero. In addition, the thermal conductivity of the Si-NW is extracted from the V-BD values under the vacuum ambient and shows good agreement with previously reported results. While the breakdown of the Si-NW does not exhibit negative differential resistance under the vacuum ambient, it interestingly shows negative differential resistance with multiple resistances in the current-voltage characteristics under the air ambient, similar to the behavior of carbon nanotubes. This behavior is triggered by current-induced oxidation, which leads to the thermo-morphic transition observed by TEM analyses. Additionally, the current-induced oxidation is favorably applied to reduce the size of a Si-NW at a localized and designated point.