This paper presents a second higher-order-mode microstrip leaky-wave antenna (LWA) for single main beam radiation. Different from its conventional second higher-order leaky-mode counterpart with two symmetric radiation beams, the proposed antenna with an array of I-shaped slots radiates only one main radiation beam at the cross-sectional center plane. By introducing a transversally loaded slot array along the microstrip line, the cross-polarization of the antenna can be largely enhanced, while a longitudinally loaded slot array interrupts the current flow and thus effectively reduces the original two radiation beams. Subsequently, an array of I-shaped slots, including transversal- A nd longitudinal-slot portions, is etched on the strip conductor and its unique characteristics are then analyzed to obtain a single main radiation beam in the center plane. To validate this proposal, a prototype of single main beam LWA with second higher-order mode is designed, implemented, and fabricated. Owing to larger leakage constant, the LWA presented in this paper radiates higher power within a shorter length in terms of guided wavelength, resulting in a higher gain. The measured results show a beam-scanning capability from 20° to 55° as the frequency varies from 5.1 to 4.6 GHz, while the maximum gain of 14.6 dBi is achieved at 4.9 GHz.