A memory system includes a memory controller and a memory device. The memory controller generates refresh commands periodically by an average refresh interval. The memory device performs a normal refresh operation and a hammer refresh operation during a refresh cycle time. The memory device includes a memory cell array including memory cells connected to a plurality of wordlines, a temperature sensor configured to provide temperature information by measuring an operation temperature of the memory cell array and a refresh controller configured to control the normal refresh operation and the hammer refresh operation. The refresh controller varies a hammer ratio of a unit hammer execution number of the hammer refresh operation executed during the refresh cycle time with respect to a unit normal execution number of the normal refresh operation executed during the refresh cycle time.