It has been a difficult problem to detect the defects in aluminum alloy friction stir welding(FSW) joints, especially those in thin plates. Recently most of the methods adopt the transverse wave or the ultrasonic phased array S-scan. However, these means cannot present all of the defects in a single picture effectively and efficiently. Ultrasonic phased array technology is used to detect the inner defects just above the upper surface of the weld by C-Scan on different sorts of defects in plates with various thickness. A mechanical device is developed to accelerate the procedure of detection. What’s more, the influence of the roughness of the FSW surface on its detectability is analyzed. The results of the experiments show that 0.3 mm-diameter side hole and 0.5 mm-diameter flat bottom holes in a 5 mm-thick flat plate can be detected effectively with the help of ultrasonic phased array.