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Read current fluctuation (ΔIread) due to random telegraph noise was measured from a cell in a NAND flash memory cell string, and its effect on threshold voltage fluctuation (ΔVth) was analyzed. Sixteen-level fluctuation (four traps) was observed in a 60-nm cell of a cell string (ΔIread/Iread of ~0.4). ΔIread increased with decreasing Lg, and ΔIread/Iread up to 0.75 was observed at 48 nm. ΔIread, ΔVth, and their relation were clearly analyzed with program/erase mode of a cell and pass cells in a string. Although ΔIread is largest when a read cell and pass cells are erased, ΔVth is largest when a read cell is erased and pass cells are programmed in a cell string. We also observed the specific noise amplitude under various conditions, such as the bit-line bias, the pass bias of unselected cells in the NAND strings, and the temperature.