In this work, a design method of a dual-frequency high-gain omnidirectional antenna is investigated and implemented. The antenna is based on the bidirectional leaky-wave antenna (Bi-LWA) theory, which can generate the desired high-gain radiation at broadside in one plane. To realize omnidirectional radiation in the other plane, the half-width microstrip line (HWMSL) was utilized. Moreover, the first and second modes of the HWMSL are both employed, this leads to the antenna operated at two separated frequencies. It is thereafter revealed that a hybrid first and second mode wave will be excited at the higher operation frequency, which is due to the high-pass property of the first mode. After that, a pin-loaded method is developed to suppress the undesired mode. The antenna is finally fabricated and measured. And the simulations and measurements both show that the predicted dual-frequency high-gain omnidirectional radiations are realized.