An operating method of a memory device, the method including, receiving a row address, determining whether an operating mode is a byte mode, counting up an access count value for the row address while ignoring a page bit, when the operating mode is the byte mode, selecting a target row hammer address among target row addresses using access count values for the target row hammer address, calculating a victim row address corresponding to the target row hammer address, and performing a target refresh operation on the victim row address.