Monolayer tungsten sulfide (WS 2 ), a desirable transition metal chalcogenide semiconductor material for the next generation of promising electronics and optoelectronics applications, has attracted great attention. However, the lack of understanding of the underlying growth mechanism in the chemical vapor deposition process inhibits the precise dimension control of WS 2 and its practical applications. Herein, we discovered that H 2 played an important role in the growth of WS 2 when tungsten trioxide (WO 3 ) was used as W precursor. It was found that a prereduction of WO 3 prior to the WS 2 growth is necessary, and the time to introduce H 2 into the growth system is crucial. The intermediate product of volatile W 18 O 49 contributed to the formation of single-crystal WS 2 only when the H 2 was introduced after the growth temperature reached 850 °C. The Gibbs free energy of the involved reactions was examined, and an intermediate-driven growth mechanism was proposed. The photodetector fabricated from the monolayer WS 2 exhibited a fast response speed and stable performance. A better understanding of the growth mechanism of WS 2 and the photodetector application demonstrated in this work offer guidelines for the study of other 2D materials.