Disclosed is an operation method of a memory device which performs a self-refresh operation. The method includes receiving a deep-sleep mode enter command from a memory controller, changing a magnitude of an internal voltage of the memory device from a first voltage to a second voltage smaller than the first voltage, in response to the deep-sleep mode enter command, and entering a self-refresh mode under control of the memory controller, and the internal voltage is maintained at the second voltage during the self-refresh mode.