Density functional total energy calculations in connection with ultrasoft pseudopotentials and generalized gradient approximation for exchange and correlation have been used to investigate (1) the energy profile for direct dissociative adsorption of GeH4 on a Ge(001) surface along two possible reaction pathways, namely an adjacent two dimers mode and a single dimer mode, and (2) the influence of their transition states on the gas-surface reactivity along these two reaction pathways. Our calculated results found that the gas-surface reactivity along the reaction pathway of the single dimer mode is slightly higher than that along the reaction pathway of the adjacent two dimers mode. We attribute this higher gas-surface reactivity to forming a 4-center ring transition state structure accompanied by a smaller distortion of the Ge–H bond within GeH4 and a smaller elongation of the GeGe dimers on the Ge(001) surface. In addition, our calculated results suggest that there should be a larger increase of gas-surface reactivity along the reaction pathway of the adjacent two dimers mode with increasing substrate temperature. This is due to the fact that there are vibrational modes of lower frequency of longer GeGe dimers on the Ge(001) surface involved in a 7-center ring transition state structure. Finally, some connections between the two transition state structures and the molecular beam experimental data are discussed.