A metamaterial-based microstrip patch antenna with a symmetrical periodic structure is proposed in this paper. Additionally, the efficiency enhancement method of the proposed symmetrical periodic antenna (SPA) is also presented using triangular slots embedded on the ground plane. The proposed antenna has a monopole-like omni-directional radiation occurred by the characteristic of composite right/left handed (CRLH) transmission line (TL). The proposed antenna resonates at 3.325 GHz in the zeroth-order frequency and 5.165 GHz in the right-handed (RH) frequency due to the symmetrical periodicity, respectively. The measured efficiency of the proposed antenna is increased about 13 % at the zeroth-order resonant modeby increasing the triangular slot length (L) from 0 mm to 5.5 mm.