New equations are proposed for stress measurements in orthotropic materials by ultrasonic birefringence technique. Intensity of stress-induced anisotropy and its principal direction are related to the applied stress through three coefficients, in contrast to only one coefficient required for isotropic materials. Experiments of ultrasonic birefringence were carried out to determine the three coefficients in a plate of aluminum alloy with a slight orthotropy originated from roll working. Anisotropy induced by uniaxial stress and rotation of the principal axis were observed in several specimens with different directions of rolling. The results are in good agreement with the curves theoretically predicted by the equations proposed here, showing the validity and usefulness of these equations for stress measurements in materials conventionally used.