Conoscopic interference pattern in pure and doped LiNbO3 (LN) has been investigated with the light propagated along z direction. Anomalous change of birefrigence is observed at about 45 ℃ as well as 75 ℃ in pure LN. The crystal changes from uniaxial crystal to biaxial crystal near these two temperatures. Similar phenomena also occur in doped LN, but the temperature ranges of the anomalies are widened. The index of refraction n0 and ne were both measured. Abrupt changes are also discovered at about 45℃ and 75℃ . The behavior of conoscopic interference at 75℃ is very similar to that of the electrooptic effect with the electric field being applied along x direction of LN at room temperature. This suggests that the anomalous change at 75℃ in LN is caused by internal electric field of about 105V/m resulting from the displacement of ions along x direction in LN.