The reaction mechanism of the fluorination process in thermal etching of Si using F compounds as etchants has been studied again in the present paper as well as in the previous paper (Tachibana et al. 1991) by using ab initio quantum-chemical techniques at the Hartree-Fock (HF) level with the 3-21 G(*) basis sets. Electron correlation effects have been estimated by using the second- and third-order Moller-Plesset perturbation methods (MP2 and MP3) with the 6-31G** basis set. We have proposed again the F-migration model for the fluorination process and adopted the following model reactions, which are larger than those adopted in the previous paper: staggered-SiH₃SiH₃ + SiH₃F → staggered-SiH₃SiH₃ + SiH₃F (1) and staggered-SiH₃SiH₂F + SiH₃F → staggered-SiH₃SiH₃ + SiH₃F₂ (2). Activation energy and heat of reaction of model reaction 1 have been estimated to be 66.4 and 0.0 kcal/mol and those of reaction 2 to be 63.5 and -9.1 kcal/mol at the MP3/6-31G**//HF/3-21G(*) level. Hence reaction 2 is considered to occur more readily than reaction 1 from the view points of both reaction kinetics and thermodynamic stability. The present study thus provids a simple picture for the F-migration process that adsorbed F atoms tend to accumulate locally in bulk silicon, which may finally lead to the formation of surface fluorosilyl layer. This result is consistent with our previous computational results and with the experimental observation reported by McFeely et al (McFeely 1986).