A memory device is provided which is capable of adjusting an operation voltage, and an application processor is provided for controlling the memory device. The memory device may include: a receiving terminal for receiving a voltage control signal from an external source, the voltage control signal being for adjusting an operation voltage level according to an operation speed of the memory device; and a voltage adjustment unit for adjusting a level of an operation voltage of the memory device in response to the voltage control signal. The level of the operation voltage is adjusted before a memory operation is performed at the operation speed corresponding to the adjusted operation voltage.