The water-soluble fluoride compound, hexafluoroboric acid sodium salt (FBNa), was coated with a polysiloxane layer by treatment with γ-nethacryloyloxypropyl trimethoxy silane (MPTS). The polysiloxane-coated FBNa was added to the experimental UDMA-based composite resin as a fluoride releasing source to regulate the fluoride releasing property. The coated FBNa-added composite resin showed approximately half the amount of the fluoride release (cummulative value for 28 days) of non-coated FBNa-added composite resin. There was no significant difference in mechanical properties (transverse strength and elastic modulus) between the coated FBNa-added composite resin and noncoated FBNa-added composite resin. The results of this study suggested that the coating of the fluoride with a polysiloxane layer could regulate the release rate of fluoride over a long duration.