Microstructural evolution of focused ion beam machined Mg nanopillars was investigated by in situ heating experiments in a transmission electron microscope. The dislocation loops generated by a Ga+ ion beam were annihilated at around half of the melting point of Mg. At a higher temperature (673 K), the sublimation of Mg occurred due to the reduced stability of Mg under the vacuum environment. In the course of sublimation, the Ga-rich liquid was formed and stabilized the structural instability of moving solid–vapor interface.