In this article, micro-scale structures of a solid part of large-thickness composites embedded with a metal adapter were studied through ultrasonic non-destructive testing and metallographic microscopy. Decay of ultrasonic intensity was observed when the metal adapter was wrapped around by 0 degree unidirectional carbon fibers reinforced prepregs. Mechanisms for the ultrasonic decay were analyzed. First, it was micro-scale air bubbles included in prepregs that caused high porosity and formed internal looseness in composites. Second, micro-cracks caused by inner stress during the curing cycle were found responsible for ultrasonic decay. Third, fiber buckling deformation and localized resin also had effects on ultrasonic waves.