In a program method of a flash memory device where memory cells within a string are turned on to electrically connect channel regions, all of the channel regions within a second string are precharged uniformly by applying a ground voltage to a first bit line connected to a first string including to-be-programmed cells and a program-inhibited voltage to a second bit line connected to the second string including program-inhibited cells. If a program operation is executed, channel boosting occurs in the channel regions within the second string including the program-inhibited cells. Accordingly, a channel boosting potential can be increased and a program disturbance phenomenon, in which the threshold voltage of program-inhibited cells is changed, can be prevented.