In recent years, the enormous success of NAND Flash memory technology in realizing multi-gigabyte memory chips has evidently triggered a lot of difficulties concerning its cell operation, such as parasitic neighbouring cell coupling, FN-tunnelling statistics, Vt distribution widening by RTN, et al. In this paper, two kinds of phenomena are shown. One is the increase of the interface state density after write/erase cycles, which will degrades the sub-threshold swing (SS) of the memory cell in the NAND string. The other is the increase of the programmed Vt distribution after programming, which also reduces the cell operation margin. It is revealed that Vt distribution widening closely depends on the floating gate doping concentration of Phosphorus. These phenomena become more serious as cell size smaller.