Program method of flash memory capable of compensating reduction of read margin between states due to hot temperature stress

A program method of a flash memory, capable of compensating read margin reduction due to hot temperature stress(HTS) is provided to secure the read margin between adjacent states in spite of electric field coupling/F-poly coupling and HTS, by secondarily programming memory cells to have a threshold voltage equal to or higher than a verify voltage of a first program operation. According to a program method of a flash memory device comprising a plurality of memory cells to store multi-bit data indicating one of plural states, memory cells selected to have one of the states are programmed with the multi-bit data. The programmed memory cells belonging to a certain region of a threshold voltage distribution where the programmed memory cells are distributed are detected. The certain region of each state is selected by one of a first verify voltage and a read voltage and a second verify voltage, where the second verify voltage is higher than the first verify voltage and the read voltage is lower than the first verify voltage. The detected memory cells are programmed to have a threshold voltage equal to or higher than the second verify voltage corresponding to each state. Therefore, the read margin between the adjacent states reduced due to HTS is increased.