The viscosity is measured in different lithium niobate melts with congruent and stoichiometrical composition in the temperature range of 1500-1620 K. A larger Li2O content reduces the viscosity of the LiNbO3 melt. The viscosity of the stoichiometrical melt depends linearly on the temperature, but the viscosity of the congruent melt departs from a linear relationship. In the range of measured temperatures there is one activation process for the stoichiometrical melt, the activation energy of the viscous flow being E-b = 52.56 kJ/mol. For the congruent melt, there are two activation energies of viscous flow, the higher-temperature zone is E-b = 43.03 kJ/mol and the lower temperature zone 20K above the melting point E-b = 72.6 kJ/mol. The abrupt increase of the activation energy of the viscous flow may stem from the change of the structure of the congruent composition melt close to the melting point.