In this article, poly-Si gate-all-around (GAA) field effect transistors (FETs) using sidewall damascene method are successfully demonstrated. By manipulating the stress which is imposed by nitride layer, the crystallinity of poly-Si channels can be modified easily by changing the thickness of nitride layer. The better crystallinity of the devices with 60 nm top nitride is attributed to larger average grain size and fewer defects, leading to higher field-effect carrier mobility compared to 40 and 80 nm top nitride layer devices. Both n-type and p-type devices exhibit superior electrical characteristics including higher on-state current of 40 μA/μm (n-type) and 26 μA/μm (p-type), steep subthreshold swing of 82 mV/dec. (n-type) and 104 mV/dec. (p-type), an extremely low drain-induced barrier lowering (DIBL) of 4.6 mV/V (n-type) and 16.6 mV/V (p-type), and high Ion/Ioff current ratio larger than seven orders of magnitude. The thermal stability and gate stress reliability measurement of sidewall damascene GAA nanowire poly-Si devices were also investigated. With better crystallinity, electrical characteristics of GAA nanowire poly-Si devices degrade less under same elevated temperature condition. Devices characteristics remain unchanged after long gate stress time. This simple fabrication process makes it a potential candidate for future three-dimensional integrated-circuit (3D-IC) and low-cost Internet of Things (IoTs) applications.