A method of operating a memory device includes writing cell data having one of at least three states to a memory cell; amplifying a voltage level of a bit line connected to the memory cell; determining that the cell data is in a first state when the voltage level of the bit line sensed at a sensing point is equal to or greater than a first reference voltage; determining that the cell data is in a second state when the voltage level of the bit line sensed at the sensing point is equal to or less than a second reference voltage which has a lower voltage level than the first reference voltage; and determining that the cell data is in a third state when the cell data is not in the first or second states.