A memory system may include a cache memory, a nonvolatile memory, a write back wait queue, and a controller. To evict an eviction cache entry including a target transaction ID from the memory cache to the nonvolatile memory, the controller performs write back operations on cache entries respectively corresponding to waiting entries at a head of the write back wait queue until a waiting entry including the target transaction ID arrives at the head of the write back wait queue, and then performs a write back operation on the eviction cache entry.