Electrical characteristics with various program temperatures (T_PGM) in three-dimensional (3-D) NAND flash memory are investigated. The cross-temperature conditions of the T_PGM up to 120 °C and the read temperature (T_READ) at 30 °C are used to analyze the influence of grain boundaries (GB) on the bit line current (I_BL) and threshold voltage (V_T). The V_T shift in the E-P-E pattern is successfully decomposed into the charge loss (ΔV_T,CL) component and the poly-Si GB (ΔV_T,GB) component. The extracted ΔV_T,GB increases at higher T_PGM due to the reduced GB potential barrier. Additionally, the ΔV_T,GB is evaluated using the Technology Computer Aided Design (TCAD) simulation, depending on the GB position (X_GB) and the bit line voltage (V_BL).