The 3-D stacking of multiple layers of NAND using thin-film transistor (TFT) devices is widely accepted as the next step in continuing NAND Flash scaling. Low mobility and reliability problems are two well-known concerns regarding TFT devices. However, another important implication of using TFT devices is that the Vt variation induced by randomly distributed grain boundaries degrades the array performance. In this paper, an extensive TCAD simulation was conducted to systematically investigate how grain boundary generated traps affect NAND Flash devices. Minimizing the density of grain boundary traps is crucial for array performance. In addition, optimal gate control ability reduces the impact of grain boundaries. Thus, using double gate architecture in vertical gate 3-D NAND is favorable. Furthermore, when pitch is scaled in the future, device exhibiting smaller channel thickness should be used to increase the gate control.