This letter presents an improved fundamental EH0-mode microstrip leaky-wave antenna (MLWA) with a transversal single beam by periodically loading a set of shorting pins and U-shaped slots along the longitudinal direction. The original EH0-mode MLWA is constructed by periodically loading a set of shorting pins underneath the microstrip line, but it, unfortunately, radiates symmetric dual beams in the transversal plane. By etching a U-shaped slot in proximity to a shorting pin inside a periodic unit, the adjacent electric field around the shorting pin is interrupted so as to enhance the θ-polarized single beam in the longitudinal plane and weaken the dual symmetric φ-polarized dual beams in the transversal plane. An equivalent model is at first established to demonstrate the unique radiation principle of the proposed single-beam MLWA. Next, the effects of the size, shape, and position of a U-shaped slot on the radiation beam are analyzed under fixed pins. In final, a prototype MLWA structure with the improved transversal single-beam radiation performance is designed, fabricated, and measured. The simulated and measured results are found in good agreement with each other, thus validating that the proposed slot-loading technique is indeed useful for the single-beam design of EH0-mode pin-loaded MLWAs.