This paper presents an improvement of the memory cell reliability by the memory cell V_TH optimization of the ferroelectric (Fe)-NAND flash memory. The effects of the memory cell V_TH on the reliability of the Fe-NAND flash memory are experimentally analyzed for the first time. The reliability is evaluated by the measured V_TH shift due to the read disturb, program disturb and data retention. Three types of Fe-NAND flash memory cells, a positive, zero and negative V_TH memory cell, are defined on the basis of the memory cell V_TH. The middle of V_TH of programmed and erased states is 1V, 0V and -0.3V in a positive, zero and negative V_TH memory cell, respectively. The V_TH shift of the positive, zero and negative V_TH memory cells show similar characteristics in the program/erase and the V_PASS and V_PGM disturbs because the external electric field is so high that the internal depolarization field does not affect the V_TH shift. On the other hand, in the data retention, the V_TH shift of the three types of V_TH memory cells show different characteristics. The reliability of the Fe-NAND flash memory is best optimized in the zeroV_TH memory cell. In the proposed zero V_TH Fe-NAND flash memory cell scheme, the measured V_TH shift due to the read disturb, program disturb and data retention decreases by 32%, 24% and 10%, respectively, compared with conventional positive V_TH Fe-NAND flash memory cell scheme. Contrarily, in the negative V_TH memory cell, the V_TH shift during the data retention is 0.49V and unacceptably large because of the depolarization field. The conventional positive V_TH memory cell suffers from a sever read and program disturb. The measured results are drastically different from those of the conventional floating-gate NAND flash memory cell where the negative V_TH memory cell is most suitable in terms of the reliability.