The reactivity of cyclosiloxane with 3,3,4,4,5,5,6,6,6-nonafluorohexyl group and its application to fluorosilicone synthesis were studied. In contrast to the polymerization of the commercially available 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane (CF3-D3), the polymerization of 1,3,5-tris(3,3,4,4,5,5,6,6,6-nonafluorohexyl)-1,3,5-trimethylcyclotrisiloxane (C4F9-D3) with sodium hydroxide yielded 1,3,5,7-tetrakis(3,3,4,4,5,5,6,6,6-nonafluorohexyl)-1,3,5,7-tetramethylcyclotetrasiloxane {[C4F9CH2CH2(CH3)SiO](4) (C4F9-D4)} as the major product, which was the result of the depolymerization by a backbiting mechanism. On the other hand, the polymerization Of C4F9-D3 with trifluoromethanesulfonic acid yielded a polymer with good reproducibility, although the distribution of molecular weight tended to be bimodal. C4F9-D3 was also successfully copolymerized with octamethylcyclotetrasiloxane (D4). The properties of the obtained copolymers changed almost linearly with the amount of [C4F9C2H4(CH3)SiO] units, except for the surface tension. The surface tension of the copolymer containing more than 50% [C4F9C2H4(CH3)SiO] units by weight was almost as low as that of the homopolymer. When the copolymer containing 50% [C4F9C2H4(CH3)SiO] units by weight was examined for an antifoaming agent, it worked as effectively as the homopolymer in the toluene foaming system.