The oxidation mechanism of dimethylamine borane (DMAB) as a reductant fur an electroless deposition process was studied by an ab initio molecular orbital method. Two types of reaction pathways, via either three-coordinate borane molecules obtained by primary dehydrogenation reactions, or five-coordinate ones by primary additions of OH- for the oxidation of the DMAB, were examined. While the former pathway corresponds to the general oxidation mechanism of the reductant proposed by Meerakker, the present theoretical results support the latter one. Furthermore, it was clarified that an electron emission occurs when OH- adds to the four-coordinate compounds, which agrees with the Meerakker＇s mechanism. Results of the normal-mode analyses showed that the five-coordinate compounds are the transition states, The optimized geometries of monoanion five-coordinate molecules are nearly bipyramidal. The charge and spin-population analyses indicated that the axial bondings in the five-coordinate compounds are stabilized by the three-center three- or four-electron bondings. During the oxidation reaction of the DMAB, the change in the net charge of B is much smaller than that in the formal oxidation number, which is due to a great covalence. The existences of the five-coordinate borane molecules, which are first clarified by the present study, could be the key points of the catalytic activities of the deposited metals.