Controllable synthesis of large domain, high-quality monolayer MoS2 is the basic premise both for exploring some fundamental physical issues, and for engineering its applications in nanoelectronics, optoelectronics, etc. Herein, by introducing H2 as carrier gas, the successful synthesis of large domain monolayer MoS2 triangular flakes on Au foils, with the edge length approaching to 80 mm is reported. The growth process is proposed to be mediated by two competitive effects with H2 acting as both a reduction promoter for efficient sulfurization of MoO3 and an etching reagent of resulting MoS2 flakes. By using low-energy electron microscopy/diffraction, the crystal orientations and domain boundaries of MoS2 flakes directly on Au foils for the first time are further identified. These on-site and transfer-free characterizations should shed light on the initial growth and the aggregation of MoS2 on arbitrary substrates, further guiding the growth toward large domain flakes or monolayer films.