A kind of metamaterial structure is designed. Unlike conventional substrate material, the new structure is composed of a double-layer metallic holes array interacting with high power microwave. According to S-parameter retrieval methods, the metamaterial exhibits the near zero refractive index which has an influence on the E-field distribution. Impacts on the antenna design are discussed and experimentally demonstrated while the Panasonic 2M244-M1 magnetron is employed as microwave source. The half-power width get a drastic reduction from 28° to 15° and the antenna gain is improved by 1.7 dB.