The reaction of CH3 + HCNO was theoretically studied by both density functional theory and molecular orbital calculations and analyzed by quantum statistical methods. The potential energy surface was constructed at the UCCSD(T)//B3LYP/6-311++G(3df,2p) + ZPE level. Four entrance channels are opened relating to the interaction of the CH3 radical with each of the four atoms of the HCNO molecule, giving rise to eighteen different product sets. The predicted geometries and heats of reaction are in good agreement with available experimental data. The major pathways involve addition of CH3 to the C atom of HCNO with a small energy barrier of ~4 kcal/mol forming both Z and E-HC(CH3)NO (IS1 / IS1b) intermediates lying 45 or 44 kcal/mol, respectively, below the reactants. The nascent intermediates can collisionally be deactivated and subsequently decompose into H + CH3CNO, or isomerize prior to decomposition giving other products. Kinetics calculations covering the temperature range of 400 – 2500 K, under pressures of 7.6 × 10-1 - 7.6 × 105 Torr for N2, He, and Ar as the third bodies show that at 760 Torr N2, the adducts including both IS1 and IS1b are the major products at temperature below 600 K, while H + CH3CNO and CNO + CH4 are the major products at T ≥ 1500 K. The total high-pressure rate coefficient can well be expressed by the following 3-parameter equation: k(T) = 7.75 × 10-16  T1.69 exp (-1480 K/T) cm3 molecule-1 s-1.