In order to verify the effects of polycrystalline Si (poly-Si) body thickness scale-down on read operation in 3D NAND flash memory which has tube type thin body, TCAD simulations and the measurements of fabricated devices are performed. I-D-V-G characteristics and transient drain current behaviors are investigated in 3D NAND devices with various body thicknesses and grain sizes. It has been known that drain current undershoot/overshoot is observed in poly-Si channel devices by falling/rising step gate bias. These phenomena are strongly related with transient of potential barrier height due to slow capture/emission rate of poly-Si grain boundary traps. As the body thickness decreases with the same grain size, the transient current instability, on-state current, and subthreshold-swing are improved. When the grain size is increased with the same body thickness, the transient current instability, on-state current, and subthreshold-swing are improved.