The transmit tance curve of the LiNb3O8 crystal is determined. It is clear that the existence of oxygen vacancy is the basic reason of blackening of the crystal and the black colour can be bleached by annealing. It is shown that the cause of the hydrogen bond formation is a chemical reaction of the atmospheric water vapor with the melt. Vacumm annealing is a method to eliminate the hydrogen bond. Crystal growth in pure oxygen atmosphere is suggested. Three principle indices of the LiNb3O8 crystal and the directions of corresponding principle axes with respect to crystallographic axes are determined. And the refractive index of the LiNb3O8 crystal is compared with that of the LiNbO3 crystal. The relationship of the principle indices with the crystal structure is discussed.