This paper introduces nanoscale gate-all-around (GAA) n-MOS polycrystalline silicon thin-film transistors (poly-Si TFTs) by using microwave annealing (MWA). Experimental results of MWA GAA poly-Si TFTs indicate high performances with subthreshold swing (SS) of 105 mV/dec., and ION/IOFF ratio of 107 A/A. MWA reveals sufficient dopant activation efficiency, which is equivalent to rapid thermal annealing. Additionally, the short channel effect is reduced owing to the low-temperature process of MWA and superior gate control of the GAA structure. Moreover, using NH3 plasma treatment further improves the device mobility, ION/IOFF ratio, and SS. Importantly, the proposed MWA GAA poly-Si TFT with its high performance and low-temperature process is highly promising for advanced 3-D ICs.