Two-dimensional (2D) transition-metal dichalcogenide (TMD) alloys have raised great interest since these 2D alloys can modulate the band gap of TMDs and thus have promising applications in nanoelectronics and optoelectronics. Here, we have grown large-size ternary Mo1-xWxS2 (x = 0–0.5) monolayers on SiO2/Si substrates based on our previous achievement on the growth of large-size MoS2 monolayers using the same confined-space chemical vapor deposition (CVD) method. The as-grown Mo1-xWxS2 monolayers are uniformly distributed on the whole growth substrate and their sizes are all quite large with an average size of 300 µm and a maximum size of even up to 500 µm. We attribute such large size to the confined-space CVD scheme, where a stable local growth environment can be preserved and the target substrate surface can be kept clean due to the protection effect of the assistant substrate and thus the nuclei density can be reduced greatly. The as-grown Mo1-xWxS2 monolayers possess high quality and uniformity as reflected by Raman, PL spectra and corresponding mapping figures. The change in W content can modulate the bandgap. Furthermore, large-area ternary W1-xMoxS2 (x = 0–0.5) monolayers can also be achieved just by simply swapping the position of MoO3 and WO3 sources and making a slight experimental adjustment. Such band gap engineering of atomically thin 2D TMDs is useful for their future applications in photoelectronics and photonics, and further proves the usefulness of our confined-space CVD method on the bottom-up growth of large-scale 2D TMDs and TMD alloys.