The　detection　of　crack　initiation　in　steel　Q235,　16MnR　and　20　g　was　carried　out　by　metal　magnetic　memory　(MMM)　technique.　Fifty　specimens　of　welded　and　base　metal　were　tested.　Both　MMM　testing　and　metallographic　examination　were　done　to　them　when　unloaded　at　the　different　phase　of　high-cycle　fatigue　testing,　and　hence　MMM　signals　before　and　after　crack　initiation　could　be　recorded.　The　values　of　magnetic　intensity　gradient　were　calculated,　and　whose　critical　value　was　determined　and　proposed　for　early　defects　detection.　The　results　show　that　the　magnetic　intensity　(Hp)　curve　became　concave　responding　to　the　occurrence　of　stress　concentration,　and　its　gradient　(dHp/dx)　increased　greatly;　at　the　critical　dHp/dx,　no　change　occurred　to　the　microstructure,　but　beyond　the　critical　value,　dHp/dx　increased　suddenly　and　a　large　number　of　intragranular　slips　were　found　in　the　microstructure.　Three　different　kinds　of　materials　have　different　critical　values　of　magnetic　intensity　gradient.