The first amorphous InGaZnO (a-IGZO) gate-all-around (GAA) nanosheet FET is demonstrated. All of the process temperatures are below $300^{\circ}\mathrm{C}$, showing great back-end-of-line (BEOL) compatibility. The channel release is achieved by sophisticated reactive-ion etch (RIE) with extremely high etching selectivity of the SiN sacrificial layer over the a-IGZO channel. A novel composite field oxide (FOX) is exploited to form an etching stop layer and avoid gate leakage. The device with gate length (L) of $52\mathrm{~nm}$ shows $\mathrm{I}_{\text{off}}\lt 10^{-7}\mu\mathrm{A}/\mu\mathrm{m}$ (detection limit), high $\mathrm{I}_{\mathrm{on}}/\mathrm{I}_{\mathrm{off}}\gt 1.3\times 10^{8}$, the enhancement mode with the positive threshold voltage $(\mathrm{V}_{\mathrm{T}})$ of $3.5\mathrm{~V}$, and the clear saturation region in the output characteristic. Moreover, the smallest SS of $61\mathrm{mV}/\mathrm{dec}$ among all oxide semiconductor nanowire /nanosheet devices is achieved with the gate length of $150\mathrm{~nm}$.