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Gallium oxide (Ga 2 O 3 ) has attracted extensive attention as a potential candidate for low-dimensional metal-oxide-semiconductor field-effect transistors (MOSFETs) due to its wide bandgap, controllable doping, and low cost. The structural stability of nanoscale Ga 2 O 3 is the key parameter for designing and constructing a MOSFET, which however remains unexplored. Using in situ transmission electron microscopy, we reveal the size-dependent phase transition of sub-2 nm Ga 2 O 3 nanowires. Based on theoretical calculations, the transformation pathways from the initial monoclinic β-phase to an intermediate cubic γ-phase and then back to the β-phase have been mapped and identified as a sequence of Ga cation migrations. Our results provide fundamental insights into the Ga 2 O 3 phase stability within the nanoscale, which is crucial for advancing the miniaturization, light weight, and integration of wide-bandgap semiconductor devices.