A theory for light extinction of LiTaO3in terms of the nonuniformity of its natural birefringence is presented. The local gradient of the natural birefringence is obtained by observing the fringe pattern appearing on the crystal between crossed polarizers, and the extinction ratios are measured using a He&sngbnd;Ne laser. Agreement between the expected and the observed values is quite good, and simple technique for evaluating the crystal quality for light modulation is established.