Poly-Si GAA FETs using sidewall damascened method are successfully demonstrated. By manipulating the stress imposed by nitride layer, crystallinity of poly-Si can be modified by changing the thickness of top nitride. Devices with larger grain size and fewer defects lead to superior electrical characteristics. Hot carrier and gate stress reliability of devices were then investigated. With better crystallinity, electrical characteristics degrade less under hot carrier stress due to less electric field enhancement. On the contrary, degradation of gate stress reliability is less sensitive to different crystallinity level. This is owing to the smaller activation energy of hot carrier effect making it more sensitive to crystallinity. With better crystallinity, poly-Si GAA nanowire FETs possess not only better electrical characteristics but also degrade less under stressing.