Abstract: Ultrasonic phased array inspection technology was used to detect the defects in friction stir welded seam with different thicknesss. The inner defects were precisely quantified and represented by changing the scanning angle,focus depth and focus size of phased array. The morphology of the defects was observed by dissecting the welded seam with defect signal. The results show that the inner defects can be quickly determined with higher detection sensitivity probe. The position,size and distribution of the defects can be precisely detected through three views obtained by different scanning modes. The shape and peak value of echo signal were determined by the defect shape. The echo signal of little hole defect was a sharp peak and the peak value was large. The echo signals of aluminium coating and loose defects were multiple-cluster wave. In S-scan,the lightspots of loose defect were more intensive and distributed more widely than aluminium coating defect.