The reaction mechanisms of met hyli dyne radical (CH) with formaldehyde have been studied theoretically by means Density Functional Theory (DFT) calculations using BH and HLYP method with the basis set aug-cc-pvdz and aug-ccpvtz. The results indicate that activation energy barriers at the initial stage of the reaction is 20.3 kJ/mol. The possible pathways for the formation of 4 products such as CH2 + HCO, C2H2 + OH, H2CCO + H and CH3 + CO has been discussed in detail. Theoretical rate constants of the CH3 + CO and H2CCO+H paths for the range 298-1273 K were determined and present results show that at the range of 298-673 K, the rate constant is very low and products of CH3+CO can best be favorable. However, the rate constant is much faster since at 673 K (k = 1.93 x 10 exponet -2 s exponent -1) and products of H2CCO+H is predicted to be more dominant.