The reflection spectrum of LiNbO3 in the range of 80cm-1-2800 cm-1 is measured by means of a 170SX Fourier transformation infrared spectrometer. The optical constant spectra of LiNbO3 in the same range are calculated rapidly and accurately using the integral form of the Kramers-Kronig relation. The transversal frequencies wT, longitudinal frequencies wL and high-frequency dielectric constants ε∞ of A1 and E modes can be obtained directly from the spectra. The oscillator strength is calculated and the mutual coupling between these oscillators are discussed. The 670om-1 mode Is recognized as a fundamental frequency of the E mode.