We present a dual-mode microstrip leaky-wave antenna (LWA), which is characterized by radiating a single beam or dual symmetrical beam. Different from the conventional single-mode microstrip LWA, the designed antenna is based on the odd mode and the even mode of the employed shorted microstrip line. The working principle of these two modes is fully studied by analyzing the electric-field distributions and their dispersion curves. Additionally, it can be found that the odd mode and the even mode are capable of realizing different radiation patterns. Furthermore, a mode translator is particularly designed in order to efficiently excite the concerned two modes. Finally, the presented antenna is simulated, fabricated, and measured, and the simulated results agree well with the measurements. The tested far-field results show that the designed dual-mode LWA realized single-beam scanning from 39 to 64 when the frequency ranges from 4.5 to 5.0 GHz and dual symmetrical beam scanning from 44 to 60 as the frequency varies from 4.5 to 4.9 GHz, respectively.