Provided is a program method of a multi-bit memory device with memory cells arranged in rows and columns. The program method includes a programming each memory cell of the first group of memory cells to a state within a first group of states according to a verify voltage level of a first group of verify voltage levels within a first range of levels, and programming each memory cell of the second group of memory cells to a state within a second group of states according to a verify voltage level of a second group of verify voltage levels within a second range of levels. The lowest verify voltage level in the second range of levels is higher than the highest verify voltage level in the first range of levels. A first voltage difference between adjacent verify voltage levels within the first range of levels is different from a second voltage difference between the highest verify voltage level of the second group of verify voltage levels and the lowest verify voltage level of the third group of verify voltage levels.