This article presents a dual-frequency high-gain horizontally polarized omnidirectional (HPO) antenna with the leaky-wave concept. The first and second modes of the half-width microstrip line (HWMSL) are investigated and it is found that these two modes can be employed to design a dual-frequency HPO bidirectional leaky-wave antenna (Bi-LWA). Moreover, it is revealed that the first mode would be unexpectedly excited at the operation frequency of the second mode ( $f_{2}$ ). Therefore, shorting pins are periodically loaded underneath the HWMSL, so as to create a stopband for the first mode at $f_{2}$. With resort to the transmission line model of this pin-loaded HWMSL, the lower and upper cut-off frequencies of the stopband are explicitly derived. Under the guidance of these formulas, one can effectively control the position of the stopband, so as to suppress the undesired mode at maximum. The internal electric fields of a uniform and a pin-loaded HWMSL Bi-LWA are extracted, illustrating that the first mode component has been efficiently reduced at $f_{2}$. A prototype is finally realistically fabricated and measured for verification. The simulations and measurements both show that the desired HPO radiation patterns are realized at the designed two frequencies.