Abstract <p>This study derives the relationship between ultrasonic velocity and stress in orthogonal fiber-reinforced composites under biaxial stress conditions. By calibrating the stress coefficients in two directions, the stress values in the composite plate can be obtained using two time-of-flight measurements. The calibration of stress coefficients requires determining the first critical incident angle. Thus, micromechanics methods were used to calculate the stiffness matrix of the composite material, which was then input into a finite element model to simulate the ultrasonic velocity in the orthogonal composite material. This process determined the incident angle capable of exciting the critical refracted longitudinal wave, namely the first critical incident angle. Rectangular specimens of orthogonal composites were manufactured from glass fiber-reinforced composite plates with bidirectional layup, and uniaxial tensile tests were conducted to verify the accuracy of the first critical incident angle and to calibrate the stress coefficients. It was found that the normal stress in the fiber direction results in a decrease in the ultrasonic velocity in both the fiber and perpendicular directions. To verify the accuracy of ultrasonic stress measurement, uniaxial tensile specimens with a central hexagonal area under biaxial stress conditions and standard biaxial tensile specimens were specially designed. The experimental results for both specimens showed that the ultrasonic stress measurement results were in good agreement with the stress measured using strain gauges, confirming the accuracy and practicality of the ultrasonic stress measurement method<i>.</i></p>