Electrical　Discharge　Machining　(EDM)　is　the　machining　technique　of　discharging　corrosion　of　the　material　surface.　The　effect　of　the　EDM　pulse　current　on　the　micro　structure　and　properties　of　the　P20　specimen　surface　were　investigated.　Three　layers　were　observed　on　the　specimen　surface　after　discharging:　ablation　layer,　molten　layer　and　recrystallized　layer.　The　experimental　results　show　that　with　the　increment　of　the　pulse　current,　the　thickness　of　the　molten　layer　and　recrystallized　layer　increased.　The　contents　of　the　oxygen　and　carbon　on　the　outside　of　the　molten　layer　were　higher　than　the　base　metal　for　the　diffusion　during　the　machining.　The　micro　hardness　reduced　from　the　out　layer　to　the　base　metal　after　the　heat　treatment,　indicating　the　mechanical　properties　changing　of　the　machining　surface.　In　the　molten　layer,　micro　fracture　is　caused　by　the　residual　thermal　stress　during　the　machining.　©　(2014)　Trans　Tech　Publications,　Switzerland.