A HOBt-based immonium-type compound,(1H-benzotriazol-1-yloxy)-N,N-dimethyl methaniminium hexachloroantimonate (BOMI), was synthesized and successfully applied to the synthesis of various oligopeptides with good yields. The estimation of racemization and the influence of several reaction parameters such as solvents, bases and temperature were studied by HPLC using a model reaction. It was found that the reactivity of BOMI was much higher than that of HOBt-based phosphonium- and uronium-type coupling reagents. Moreover, its racemization was lower than that of other HOBt-derived coupling reagents. The effectiveness of BOMI was also demonstrated by the synthesis of Leu-enkephalin both in solution and in the solid-phase.