A gain-enhanced EH1-mode microstrip leaky-wave antenna (MLWA) with periodical loading of shorting pins to improve the radiation gain is proposed. Owing to the equivalent shunt inductive effect of the periodically installed shorting pins, the cut-off frequency of the proposed pin-loaded EH1-mode microstrip line (MSL) is effectively tuned up; in other words, the lateral electric dimension of the proposed MLWA is effectively enlarged. Therefore, the distance of a pair of in-phase equivalent magnetic-current line sources in two sides of the MSL is widened, resulting to increase the radiation gain of the proposed MLWA. For the sake of theoretical explanation, an analytical model is built up for analysis. Then, numerical investigation is executed to reveal the effect of pins position on the tuning ratio of both the EH1-mode cut-off frequencies and strip widths. After extensive analysis, a pin-loaded EH1-mode MLWA under width tuning ratio of 1.78 and periodicity of 10.0mm is designed, fabricated, and tested. Simulated and measured results both demonstrate that the radiation gain could be successfully raised up from 10.1dBi (conventional EH1-mode MLWA) to 13.1dBi (the proposed pin-loaded one) at the same operating frequency of 4.0GHz.