Many metamaterials, including metal wire grid, was invented to enhance the performance of the antenna in the last decade. A novel zero index metamaterial (ZIM), consisting of double metal layers with periodic circular perforation cells, was proposed and studied for horn antenna loading in this paper. The simulation model of array loaded with ZIM was constructed and optimized by CST software. The results indicate that array loaded with ZIM shows an enhanced performance at the resonant frequency and could be used for antenna loading application. An experimental study was also carried out to verify the design of ZIM. The measured resonant frequency of ZIM shows a good agreement with the results of the derived analytical formula. A maximum of 2.3 dB gain enhancement and 0.88 aperture efficiency at 9.3GHz was observed according to the measurement. Compared with typical standard horn with the same aperture, the tapering length of array loaded with ZIM was reduced by half, and the aperture efficiency demonstrates more than 70% increase at the resonant frequency in the meantime.