High-quality and large-scale growth of monolayer molybdenum disulfide (MoS2) has caught intensive attention because of its potential in many applications due to unique electronic properties. Here, we report the wafer-scale growth of high-quality monolayer MoS2 on singlecrystalline sapphire and also on SiO2 substrates by a facile metal-organic chemical vapor deposition (MOCVD) method. Prior to growth, an aqueous solution of sodium molybdate (Na2MoO4) is spun onto the substrates as the molybdenum precursor and diethyl sulfide ((C2H5)(2)S) is used as the sulfur precursor during the growth. The grown MoS2 films exhibit crystallinity, good electrical performance (electron mobility of 22 cm(2)center dot V-1 center dot s(-1)) and structural continuity maintained over the entire wafer. The sapphire substrates are reusable for subsequent growth. The same method is applied for the synthesis of tungsten disulfide (WS2). Our work provides a facile, reproducible and cost-efficient method for the scalable fabrication of high-quality monolayer MoS2 for versatile applications, such as electronic and optoelectronic devices as well as the membranes for desalination and power generation.