The overall gas phase decomposition mechanism of tetraenopentyl zirconium precursor (Zr[CH2C(CH3)3]4) for the chemical vapor deposition of zirconium carbide thin films was investigated by using computational thermochemistry. Density functional theory (DFT) and harmonic vibrational frequency calculation were used to generate thermodynamic properties at each reaction step, based on which thermodynamic or kinetic preference of a reaction pathway was evaluated. While the preference of γ-hydrogen abstraction of neopentane over α-hydrogen abstraction was confirmed in the initial stage of ZrNp4 decomposition, they turned out to be competing instead of the dominant preference of γ-hydrogen abstraction. Methane formation at three subsequent reaction steps was explained by β-methyl migration, and the following α-hydrogen abstraction of methane based on the suggestion that α- and γ-hydrogen abstractions of neopentane are competing kinetically in previous reaction steps. Computational thermochemistry showed a possibility as a general tool to anticipate the gas phase decomposition mechanism of a precursor in chemical vapor deposition.