A　unified　macroscopic　continuum　model　is　proposed　to　calculate　the　long-wavelength　optical　phonons　of　single-walled　boron　nitride　nanotubes.　The　effect　of　the　polarity　in　the　boron　nitride　nanotubes　on　optical　vibrations　is　considered　by　this　model.　We　obtain　the　dispersion　relation　of　the　optical　phonons.　The　avoided　crossing　between　longitudinal　and　transverse　optical　phonon　branches　arises　from　the　indirectly　coupling　between　different　phonon　branches.　The　diameter　dependence　of　the　optical　phonon　frequency　is　derived.　Our　results　are　in　agreement　well　with　the　experiment　ones　[R.　Arenal,　A.C.　Ferrari,　S.　Reich,　L.　Wirtz,　J.-Y.　Mevellec,　S.　Lefrant,　A.　Rubio,　A.　Loiseau,　Nano　Lett.　6　(2006)　1812].　(C)　2008　Elsevier　B.V.　All　rights　reserved.