The mechanisms of insertion of O-H bond of Rh( Ⅱ ) -methylene carbene into methanol and ethanol were studied by using B3LYP functional both in gas phase and in CH2Cl2. The formation of free alcoholic oxonium ylides is found to be impossible. Alcoholic oxonium ylide are formed as the intermediates before both the stepwise and the concerted transition states of insertion of O-H bond of Rh( Ⅱ ) -methylene carbene into methanol and ethanol. With regard to the mechanisms of insertion of O-H of Rh( Ⅱ ) -methylene carbene into alcohols, analysis of the energy barriers of the two mechanisms indicate that the stepwise mechanism is more plausible than the concerted mechanism.