Atomically thin molybdenum disulfide (MoS₂) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS₂ atomic thin layers is still lacking. Here we report that wafer-scale MoS₂ thin layers can be obtained using MoO₃ thin films as a starting material followed by a two-step thermal process, reduction of MoO₃ at 500 °C in hydrogen and sulfurization at 1000 °C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS₂ films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.