Based　on　basic　principle　of　metal　magnetic　memory　(MMM),　this　paper　proposes　a　model　of　magnetic　flux　leakage　signal　of　gear　early　local　micro　cracks.　The　tangential　component　of　magnetic　flux　leakage　signal　will　peak　at　the　crack　area　while　the　normal　component　will　pass　zero　point　and　its　gradient　has　the　peak.　The　effect　of　the　crack　depth　on　the　characteristics　has　also　been　considered.　The　theory　is　then　applied　to　detection　of　micro　cracks　on　the　tooth　surface　of　an　actual　gear.　The　microscopic　analysis　results　demonstrate　the　effectiveness　of　the　magnetic　memory　testing.