A memory device includes a plurality of memory chips for writing and reading data in response to a control command and an address signal, and a control logic circuit for transferring the control command and the address signal to the plurality of the memory chips, and receiving a first command from a memory controller to perform a first operation, different from a refresh operation, on at least one of a plurality of the memory chips. The control logic circuit, in response to a refresh command, transmits the first command to at least one of a plurality of the memory chips and performs the first operation during a pre-determined refresh time interval without carrying out the refresh operation.