A memory device including: a memory cell array including memory cell rows; and a control logic circuit to perform a row, write, read, or pre-charge operation on the memory cell rows in response to an active, write, read, or pre-charge command, wherein the control logic circuit is further configured to: calculate a first count value by counting the active command and a second count value by counting the write command or the read command, with respect to a first memory cell row, during a row hammer monitor time frame; determine a type of row hammer of the first memory cell row based on a ratio of the first count value to the second count value; and adjust a pre-charge preparation time between an active operation and the pre-charge operation, by changing a pre-charge operation time point according to the determined type of row hammer.