The gas-phase reactions involved with radicals in the TMG/NH3/H2 system of MOVPE process were studied by the density functional theory (DFT) of quantum chemistry. In particular, the effects of H and NH2radicals on the pyrolysis, hydrogenolysis and adduct paths of Ga(CH3)3 (denoted as TMG) were studied. By calculating of the Gibbs energies ΔG and energy barriers ΔG*/RT of different reaction paths, the roles of radicals on the reaction paths at different temperatures were determined. The study found that when T＜683 K, TMG reacts with NH3 to generate TMG∶NH3. When T＞683 K, TMG∶NH3 decomposes back into TMG and NH3. At MOVPE condition TMG is difficult to pyrolyze directly. While with H radicals involved, TMG can be easily pyrolyzed into Ga(CH3)2(denoted as DMG), GaCH3(denoted as MMG) and Ga atom. When T＜800 K, the rate of amide reaction between TMG and NH3 is greater than the rate of pyrolysis with H radicals, the reaction is dominated by amide reaction. When T＞800 K, the rate of pyrolysis with H radicals is greater than the rate of amide reaction, the reaction is dominated by pyrolysis with H radicals. Since the energy barrier of hydrogenolysis is rather high, the reaction can be ignored. TMG and its pyrolysis products can react with NH2 radicals easily to produce amide DMGNH2. The amides can further react with H radicals and eventually generate GaNH2 as surface reaction precursors.