Abstract: The welded gear set is widely used in automobile transmission. When the automobile is running, the transmission gear bears high load torque and is frequently impacted. The gear welds must pass the nondestructive testing to ensure their quality. Up to now, the main method of automobile gear weld detection is pulse reflection ultrasonic testing. In this paper, the feasibility and superiority of phased array ultrasonic testing for automobile gears are discussed, and the testing tests for two kinds of gears are carried out. Experimental results show that this method has the advantages of high scanning speed, good imaging quality, high measurement accuracy and high detection reliability.