A memory system may include: a memory pool including a plurality of memory regions; and a controller suitable for controlling the memory pool, wherein each of the memory regions includes one or more row groups, each row group having a predetermined row group size, and wherein the controller counts the numbers of row accesses to the respective memory regions, determines row group sizes according to the row access counts of the respective memory regions, increases a representative access count of a row group including a certain row when the row is accessed, and provides a command to the memory pool to perform a target refresh operation on a target row group whose representative access count exceeds a threshold value.