Ultrathin films of LiBr were epitaxially grown on a clean Si (100)-(2×1) substrate by molecular beam epitaxy. The surface and interface phonon (Fuchs-Kliewer mode phonon) in the grown films were measured by high resolution electron energy loss spectroscopy. The intensity and frequency of the F-K phonon depends on the film thickness. The dielectric theory is found to be applicable to 3∼12 nm thick films, but it fails for thinner films.