The aromatic CC bond cleavage by a tungsten complex reported recently by Sattler and Parkin15 offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained CC bond is activated at the molecular level.16, 17 In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic CC bond in quinoxaline (QoxH) by the W-based complex [W(PMe3)4(η2-CH2PMe2)H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol−1 and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe3 ligand on the tungsten center. In our new mechanism, it is proposed that the CC cleavage occurs prior to the second CH bond addition, in contrast to Sattler and Parkin’s mechanism in which the CC bond is broken after the second CH bond addition. We find that the rate-determining step of the reaction is the ring-opening process of the tungsten complex with an activation barrier of 28.5 kcal mol−1 after the first PMe3 ligand dissociation from the metal center. The mono-hydrido species is located as the global minimum on the potential-energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable CC bond cleavage.