In this paper, a novel structure of metamaterial is proposed in order to miniaturize a rectangular microstrip patch antenna. The metamaterial is composed of two nested split octagons which are located on a 10 mmx10 mm Rogers RT/duroid 5880 with 0.7874mm thickness and dielectric constant of 2.2. A 3×5 array of such metamaterials is placed on the patch antenna substrate. By using this metamaterial in the antenna structure, the dimension of this proposed antenna is reduced compared to a simple microstrip patch antenna.