The polymerization of polydimethylsiloxane was performed and the viscosity variation of the product was monitored during polymerization. The used monomer was octamethylcyclotetrasiloxane, and H₂O and hexamethyldisiloxane was used as a termination agent for OH and tri-methyl end group. And the used catalyst was tetrabutyl phosphonium hydroxide which was inactivated effectively at high temperature after the polymerization. The viscosity of product was increased as increase of reaction time when the amount of termination agent was relatively low. But the viscosity of product was decreased as increase of reaction time when the amount of termination was relatively high. Which viscosity trend was interpreted as a competition of addition reaction and termination reaction. The monitoring of visicosity variation during polymerization was very effective to study the effect end group for polymerization.