Transition metal dichalcogenides (TMDC s ) films are potential two-dimensional materials for next-generation optoelectronic and electronic devices. Chemical Vapor Deposition, Atomic Layer Deposition, and Metal-Organic Chemical Vapor Deposition are usually used for the synthesis of the MoS 2 films by taking Mo(CO) 6 as precursors. However, the mechanism of adsorption and reaction of Mo(CO) 6 on the substrate at the molecular level are hardly to know only by the experiments. In this study, the detailed reactions of Mo(CO) 6 on α-Al 2 O 3 (0001) are investigated by density functional theory to demonstrate the growth mechanism of MoS 2. It is found that, Mo(CO) 6 could form a stable chemical bond with the Al-1 site on the surface, which has large adsorption energy and charge transfer. The decomposition of Mo(CO) 6 into Mo(CO) 3 exists in a clear order, with the top carbonyl composing firstly, follow by the two parallel carbonyls. In order to determine the stable model of Mo(CO) 3 , adsorption energy and DOS are analyzed. The E ad is À 5.65 eV and the 4d5 s orbitals of Mo atom overlap with the 2 s2p orbitals of O atom, indicating the new chemical bond is forming in 3O-Al-2 model. Finally, the LST/QST (Linear Synchronous Transit/Quadratic Synchronous Transit) method is used to search for transition states of Mo(CO) 3 sulfuration reactions with CH 3 S 2 CH 3 , H 2 S, and S 2 respectively. This paper provides a theoretical basis for the experimental preparation of MoS 2 and provides research ideas for the general synthesis of TMDCs.