A nonvolatile memory device includes a first electrode and a second electrode, and a variable resistor interposed between the first and second electrodes. The variable resistor has a critical voltage, and a resistance-voltage characteristic of the variable resistor is switched at a voltage higher than the critical voltage, so that a resistance of the variable resistor is higher at a read voltage applied after the switching of the resistance-voltage curve than at a read voltage applied before the switching of the resistance-voltage curve. Methods of operating a nonvolatile memory device include setting a plurality of write voltages higher than an initial critical voltage, assigning respective data values to states in which a resistance-voltage characteristic is switched at the write voltages, setting a read voltage lower than the initial critical voltage, and reading the data values by measuring current flowing through the variable resistor in response to the read voltage.