To obtain a clue for understanding the low-temperature thermal chemical vapor deposition reported by Hanna et al., we studied the difference of the chemical reactivities in gas-phase between Si2H6 and SiH4 with GeF4 by quantum-chemical techniques. The calculation of electronic states was carried out at the CCSD(t)//B3LYP(+ZPE) level. For the Si-Ge bond formation reaction between GeF4 and Si2H6, the activation energy is 36.77 kcal/mol and the reaction is exothermic by 17.67 kcal/mol, while for the reaction between GeF4 and SiH4 the activation energy is 54.68 kcal/mol and the reaction is endothermic by 18.43 kcal/mol. One of the products in these reactions, SiH3GeF3, is easily decomposed into SiH3F and GeF2.