The hydrogen abstraction reaction HNCO + NH --> NCO+NH2 has been studied using ab initio molecular orbital theory. The geometries of the reactants, hydrogen-bonded complex, transition state, and products have been optimized at the UMP2/6-311G** level of theory. The forward and reverse reaction potential barriers calculated accurately at the UQCISD(T)/6-311G** level of theory are 25.80 and 2.39 kcal/mol, respectively, and the heat of reaction is 20.32 kcal/mol. The minimum-energy path was calculated by the intrinsic reaction coordinate theory (IRC) at the UMP2/6-311G** level with a gradient step size of 0.05 (amu)(1/2) a(0), The changes of the geometries and normal-mode vibrational frequencies along the IRC were discussed. The energy profile along the IRC was refined by the UMP4/6-311G** and UQCISD(T)/6-311G** single-point energy calculations, The forward and the reverse reaction rate constants for the temperature range 1000-3000 K were obtained by the variational transition-state theory, The theoretical rate constants of the forward reaction are 2.7, 10.2, and 20.5 times the experimental rate constants at the temperatures 1000, 2000, and 3000 K, respectively, The reverse reaction rate constants are at least an order of magnitude greater than the forward reaction rate constants.