Through detailed calculations by density functional theory and second-order Moller-Plesset perturbation theory (MP2) to fourth-order Moller-Plesset perturbation theory including single, double, and quadruple excitations [MP4(SDQ)] methods, we investigated the oxidative addition of the B-Br bond of dibromo(trimethylsiloxy)borane [Br2B(OSiMe3)] to Pt(0) and Pd(0) complexes [M(PMe3)(2)] (M = Pt or Pd) directly yielding a trans bromoboryl complex trans-[MBr{BBr(OSiMe3)} (PMe3)(2)]. Two reaction pathways are found for this reaction: One is a nucleophilic attack pathway which directly leads to the trans product, and the other is a stepwise reaction pathway which occurs through successive cis oxidative addition of the B-Br bond to [M(PMe3)(2)] and thermal cis-trans isomerization. In the Pt system, the former course occurs with a much smaller energy barrier (E-a = 5.8 kcal/mol) than the latter one (E-a = 20.7 kcal/mol), where the DFT-calculated E-a value is presented hereafter. In the Pd system, only the latter course is found in which the rate-determining steps is the cis-trans isomerization with the E-a of 15.1 kcal/mol. Interestingly, the thermal cis-trans isomerization occurs on the singlet potential energy surface against our expectation. This unexpected result is understood in terms of the strong donation ability of the boryl group. Detailed analyses of electronic processes in all these reaction steps as well as remarkable characteristic features of [Br2B(OSiMe3)] are also provided.