Abstract: As a new technology, ultrasonic phased array has been tried to apply in the welds testing. A weld specimen was designed which included the actual defects, such as flaw of crack, air vent, incomplete fusion and incomplete penetration etc. The ultrasonic phased array system can detect the interior defects of welds clearly through controlling the parameter such as beam angle, Focal distance, Focus size and Focus length and so on. It could distinguish the location and the size of the interior defect exactly. The experiment result revealed that phased array technique had high requirement for the testing surface, and needed to make varies sample blocks.