The kinetics and mechanism for the reaction of HCO with NO occurring by both singlet and triplet electronic state potential-energy surfaces (PESs) have been studied at the modified Gaussian-2 level of theory based on the geometric parameters optimized by the Becke-3 Lee-Yang-Parr/ 6-311G (d,p) method. There are two major reaction channels on both singlet and triplet PESs studied: one is direct H abstraction producing CO+HNO and the other is association forming a stable HC(O)NO (nitrosoformaldehyde) molecule. The dominant reaction is predicted to be the direct H abstraction occurring primarily by the lowest-energy path via a loose hydrogen-bonding singlet molecular complex, ON⋯HCO, with a 2.9-kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol). The commonly assumed HC(O)NO intermediate, predicted to lie below the reactants by 27.7 kcal/mol, has a high HNO-elimination barrier (34.5 kcal/mol). Bimolecular rate constants for the formation of the singlet products and their branching ratios have been calculated in the temperature range of 200-3000 K. The rate constant for the disproportionation process producing HNO+CO, found to be affected strongly by multiple reflections above the well of the complex at low temperature, is predicted to be k (HNO) =3.08×10-12 T0.10 exp (242T) for 200-500 K, and 1.72×10-16 T1.47 exp (888T) for 500-3000 K in units of cm3 molecule-1 s-1. The high- and low-pressure rate constants for the association process forming HC(O)NO can be represented by k∞=4.42×10-11 T0.25 exp (-28T) cm3 molecule-1 s-1 (200-3000 K) and k0=7.30×10-16 T-5.75 exp (-719T) (200-1000 K) and 1.82×102 T-11.92 exp (1846T) (1000-3000 K) cm6 molecule-2 s-1 for N2 -buffer gas. The absolute values of total rate constant, predicted to be weakly dependent negatively on temperature but positively on pressure, are in close agreement with most experimental data within their reported errors.