The particularities of 3D-woven composite’s defects are observed. Automated ultrasonic immersion through-transmission technique for their non-destructive testing is offered. X-ray computed tomography (X-ray CT) application for the definition of the reasons of ultrasonic waves high attenuation in 3D-orthogonal woven composite sample is described. The analysis of defects detected with X-ray CT is shown. The suggestion about connection between quantity of projection of defects total area at the plane which is perpendicular to ultrasonic wave propagation and ultrasonic wave attenuation has been made.