In this study, a high-gain square patch antenna based on the TM₅₀ mode is proposed. Owing to the operation of the TM₅₀ mode, there is a pair of reversed-phase currents comparing with the radiating edges on the surface of the antenna. By introducing transverse slots along the central line of the patch, each of the two reversed-phase currents are interrupted to circulate with little radiation;thus, the four side-lobes of the TM₅₀ mode can be suppressed well. Simultaneously, because the reversed-phase current is weakened, the slotted antenna is superposed by three in-phase currents, which providean effect equivalent to that of three antenna elements operating under the TM₁₀ mode. By adjusting the size of the slots, the maximum gain of the antenna can reach 13.3 dB.