Abstract: In addition to detecting the macro-cracks, metal magnetic memory(MMM) testing technique can also determine the micro-damage due to stress concentration. Due to owning many advantages of portable testing device, easy operation process, and simple damage criterion, it can be applied widely. However, as a new nondestructive testing method, many aspects are still questionable and unclear. For example, the physical mechanism and the theoretical models of this method, how to provide more reliable and quantitative testing results, and any further information received from this method are all not very clear. In addition to the basic testing principle, some typical engineering applications were shown, and some discussions and future development tendency of MMMT were presented.