The present invention is directed to a capacitor having a reaction preventing layer and a method forming the same. A lower electrode of silicon is formed on a substrate. An assistance layer of metal oxide or metal nitride is formed on the lower electrode. A nitridation process is performed to enable the silicon of the lower electrode, the assistance layer, and nitrogen supplied by the nitridation process to react with one another, forming a reaction preventing layer comprising metal silicon oxynitride or metal silicon nitride. A high-k dielectric film and an upper electrode are formed on the reaction preventing layer.