The ordinary‐ray index of refraction at 5893 Å in BaTiO3was measured in the range 20°–160°C by a refined Chaulnes' method with an average error of 0.35%. Two BaTiO3crystals were studied and the results indicate a constant index of 2.368 from 20° to 105°C; above the Curie point the ordinary index increased by 1.3% to 2.398 and remained constant to 160°C.