The reaction mechanism of Rh(III)-catalyzed cross-dehydrogenative aryl-aryl coupling between benzamides and haloarenes was investigated through detailed density functional theoretical (DFT) studies in terms of regioselectivity and deuterium kinetic isotope effects (KIEs). Three possible routes including one PivO(-)-assisted reaction route and two non-PivO(-)-assisted reaction routes have been studied. The calculated results refute the proposed mechanism (without PivO(-)-assisted process) in the experimental paper and demonstrate that the PivO(-)-assisted reaction mechanism is the most favored. Meanwhile, the calculation revealed that the PivO(-) anion plays a crucial role as a proton acceptor in the C-H bond activation, especially when the second C-H activation of haloarenearene proceeds via a S(E)3 mechanism. The S(E)3 mechanism is presented for the Rh(III)-catalyzed aryl-aryl reaction for the first time. Our mechanism is evaluated by the calculations of the para-/meta-regioselectivity and KIEs. And it is found that the second activation process is the rate-determining step of the whole catalytic cycle. All these calculated properties agree well with the experiment and Glorius's proposal that the Rh(III)-catalyzed cross-dehydrogenative C-C coupling reaction proceeds by dual C-H activations. Our theoretical studies suggest that the Rh(III) complex catalyst strongly affects the mechanisms of the second C-H activation step and thus this work might provide insight into the design of new catalytic systems.