Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS₂ ) by using thermally evaporated thin films of molybdenum trioxide (MoO₃ ) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO₃ thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS₂ throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 10⁶ under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS₂ for future semiconductor device applications.