Read current fluctuation (ΔI _read ) due to random telegraph noise was measured from a cell in a NAND flash memory cell string, and its effect on threshold voltage fluctuation (ΔV _th ) was analyzed. Sixteen-level fluctuation (four traps) was observed in a 60-nm cell of a cell string (ΔI _read /I _read of ~0.4). ΔI _read increased with decreasing L _g , and ΔI _read /I _read up to 0.75 was observed at 48 nm. ΔI _read , ΔV _th , and their relation were clearly analyzed with program/erase mode of a cell and pass cells in a string. Although ΔI _read is largest when a read cell and pass cells are erased, ΔV _th 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.