A RRAM and its manufacturing method are provided. The RRAM includes a first dielectric layer formed on a substrate, and two memory cells. The two memory cells include two bottom electrode structures separated from each other. Each bottom electrode structure fills one of two trenches in the first dielectric layer. The two memory cells also include a resistance switching layer and a top electrode structure. The resistance switching layer is conformity formed on the surface of an opening in the first dielectric layer, and the opening is between the two trenches. The top electrode structure is on the resistance switching layer and fills the opening. A top surface of the first dielectric layer, top surfaces of the bottom electrode structures, a top surface of the resistance switching layer, and a top surface of the top electrode structure are coplanar.