Recently we have been investigating solvent-free synthesis of siloxanes. Using no solvent gives less load to environment, and decreases cost in chemical industry. On the other hand, acidic or basic condition may not be applied to siloxanes bearing functional substituents. Therefore, the development of synthetic methods in neutral condition is desired. In this paper, we report the solvent-free thermal condensation of siloxane from silanol bearing vinyl group and strong electron-withdrawing 4-trifluoromethylphenyl group. Furthermore, we examined thermal properties of obtained vinyl-substituted silanol by TG-DTA measurement. With these results, we could determine the temperature of thermal condensation. We also elucidated the reaction pathway by tracing the reaction with GC-MS. Finally, we propose the possible reaction pathway from silanediol to cyclotrisiloxane (D3) and cyclotetrasilxane (D4).