Provided is a memory device including a substrate, a plurality of word-line structures, a plurality of cap structures, and a plurality of air gaps. The word-line structures are disposed on the substrate. The cap structures are respectively disposed on the word-line structures. A material of the cap structures includes a nitride. The nitride has a nitrogen concentration decreasing along a direction near to a corresponding word-line structure toward far away from the corresponding word-line structure. The air gaps are respectively disposed between the word-line structures. The air gaps are in direct contact with the word-line structures. A method of forming a memory device is also provided.