The radiation characteristics of a new millimeter-wave omnidirectional circular rod antenna with double dielectric gratins are analyzed with the rigorous mode matching method. The results indicate that under the condition of the same radiation intensity, the double grating antenna has relatively smaller dimensions than that of the single one. It is undoubtedly a great importance in the case where the limitation of the weight and volume of system is strictly required. The effectiveness and reliability of the present analysis are verified by the related results given in the literature.