This paper describes a method to detect defects on stone sculptures and timber compoments of ancient architectures with an ultrasonic CT system. This system contains an outside amplifier for weak received signals, a multi-sensor fixing apparatus, and sensor diameter converters. The converters transform the diameter of sensors from 50 mm to 10 mm, to decrease the error caused by inaccuracy of the location of both the transmitters and receivers. No coupling agent is used during the detection, to avoid artificial damage to the detected objects. We analyzed three standard wood samples and obtained the images that correspond to the appearances of the samples. Satisfactory analyses of four pillars of two buildings of Western Mountain Temple were also obtained.