In this paper, it is affirmed that the absorption edge below 3.8 eV is attributable to indirect transition, the rational ownerships of three straight-line parts of spectral line are obtained by studing the a1/2-hν curve of LiNbO3 crystals with different compositions. According to the relation between LiNbO3 lattice constant and compositions, we are sure that the energy band structure change with composition. Redfield's and ours expermental results of absorption edge are explained.