1. A program method of a flash memory device, comprising: inputting a first data and a second data to a first latch and a second latch, respectively, of a page buffer coupled to memory cells, wherein the memory cells include an even memory cell coupled to an even bit line and an odd memory cell coupled to an odd bit line; pre-programming the odd memory cell by applying a pre-program voltage to a selected word line, wherein a threshold voltage of the pre-programmed odd memory cell is higher than a first verification voltage; programming the even memory cell by applying a first program voltage to the selected word line after the pre-programming of the odd memory cell, wherein a threshold voltage of the programmed even memory cell is higher than a second verification voltage; and programming the odd memory cell that is in a pre-programmed state due to the pre-programming of the odd memory cell by applying the first program voltage to the selected word line after the programming of the even memory cell, wherein the threshold voltage of the programmed odd memory cell is higher than the second verification voltage, wherein the first verification voltage is lower than the second verification voltage.