Multiple defects including fiber waviness, resin rich layers, porosity and delamination may initiate in
aerospace composite structures during manufacturing and service, and thus potentially cause structural failure. In this paper, ultrasound testing for aerospace composites with complex microstructures was investigated through both simulation and experiment. Ultrasonic testing challenges of aerospace composites were concluded as frequency-dependent inter-ply reflection, interfering reflection from multiple defects, and frequency-dependent wave distortion caused by fiber waviness. Ultrasonic B-scan imaging with both amplitude and phase information at different frequencies are proposed to achieve defect differentiation and characterization for the thick composites with fiber waviness. This research will facilitate the understanding of defect presence in the B-scan images under different frequencies and indicate the direction for improving ultrasonic testing of aerospace composites.