In order to solve the problems of narrow band and high loss related to traditional left-handed materials (LHM), a novel structure of LHM is designed based on the equivalent circuit theory and mirror-symmetry principle. The results show that, as for this structure, its effective permittivity and effective permeability are both negative, its absolute bandwidth reaches 8.9 GHz, and it possesses a low loss characteristic in the frequency range from 16.5 GHz to 25.4 GHz. Compared with those of the traditional microstrip antenna, the gain of the microstrip antenna based on this left-handed structure increases by 3.38 dB, the half power beam width reduces by 41.11°, and the effective radiated power is obviously enhanced.