Abstract: In order to establish the relationship betweenacousto-caustics and stress intensity factor under plane stress condition, anew acoustic-stress coefficient---the ratio between acoustic-path differenceand stress is defined. The new coefficient is related not only to thematerial properties but also to the coupling medium and the acoustictraveling path. The method and the steps to get the new coefficient aredescribed together with the tested results of 6061-T6 aluminum and EPTI steel.It is concluded that the new acousto-stress coefficient can be obtainedas soon asthe ratio of relative change of ultrasonic longitudinal wave to the sum ofthe principal stresses under plane stress condition is known.