The formaldehyde decomposition occurs via two reaction paths: the molecular channel that produces H2 and CO, and the radical channel that produces H and HCO. Geometry of each compound was optimized up to the QCISD/6-311 + + G(2df,2pd) level of theory, and the result agreed well with the experimental data. The energies of the best optimized geometry were calculated at the QCISD and QCISD(T) levels with 6-311 + + G(2df,2p), 6-311 + + G(2df,2pd), 6-311 + + G(3df,3p), and 6-311 + + G(3df,3pd) basis sets. The energies computed at each level laddered down toward a stable minimum when larger basis sets were employed. At the level of QCISD(T)/6-311 + + G(3df,3pd)//QCISD/6-311 + + G(2df,2pd), the reaction heats of the radical channel and the molecular channel were 85.70 and -2.35 kcal/mol, respectively, and the reaction barrier of the molecular channel was computed to be 80.97 kcal/mol.