The OH- infrared absorption band in the LiNbO3 crystal with various Li/Nb ratios and MgO contents have been measured at room temperature. The structure of the absorption band of nearly stoichiometric crystals consists of three sub-bands with slightly different peak position, their relative heights change with Li/Nb ratios. The OH- absorption band in heavily Mg-doped crystal has a structure consisting of two peaks, the shift of peak position to shorter wavelength is about 54 cm-1. Basing on the crystal structure and the model of the defect structure of the LiNbO3 crystal suggested by Abrahams and Smyth (1-2), these observations are discussed for three different structures, i.e., stroichiometric, congruent and Mg-doped crystals, respectively. The shift of the peak position of the OH- absorption band in heavily Mg-doped crystal to shorter wavelength is suggested to be caused by the Mg2+ ions getting into Nb5+ sites when the Mg-doping level was higher than the threshold level in Mg-doped LiNbO3 crystals.