Abstract: Metal magnetic memory technology (MMMT) is a new nondestructive testing method, which is an effective method to diagnose pressure vessels in early stage. MMMT can locate the stress concentration zones according to the status of magnetization exhibited by the object. MMMT and ultrasonic testing method were used to test seven groups of CNG high pressure gas cylinders and their results were analyzed and compared. The two methods all found the same danger areas. It indicated that MMMT is a new method with high reliability and efficiency to test high pressure gas cylinders.