Abstract: This paper illustrates the results of experiments in which carbon fiber reinforced epoxy laminates, honeycomb structures composed of carbon epoxy covers and paper cores, and honeycomb structures composed of aluminum alloy covers and aluminum alloy cores are tested by an ultrasonic C-scan system for finding bonding defects. The reference blocks used in the experiments have round disk shape artificial defects which simulate bonding defects. The diameters of the defects are 4 mm, 10 mm, and 16 mm, respectively. Through transmission method and water jets coupling are used. The laminates are tested with 5 MHz ultrasound and the honeycomb structures are tested with 1 MHz ultrasound. As a result, clear and accurate C-scan pictures of the defects are obtained except those in the honeycomb structure with 135 mm thick paper core. The failure is caused by the high attenuation of the honeycomb structure.