This　paper　involves　the　simulation　of　vacuum　arc　under　composite　contact.　A　three　dimensional　model　is　developed　and　we　consider　the　ionization　process　in　the　vacuum　arc.　The　species　transport　equations,　in　which　the　fraction　of　each　species　is　calculated,　provides　the　distribution　of　various　atoms　and　ions　in　vacuum　arc.　The　model　describes　ions　and　atoms　as　two　different　processes　considering　that　ions　are　influenced　by　self-generated　magnetic　field.　Atoms　play　an　inferior　role　in　the　vacuum　arc　plasma　process.　They　gather　in　an　＇atom　layer＇　whose　thickness　is　a　tenth　of　contact　gap.　Furthermore,　the　distribution　of　ion　species　is　closely　related　to　the　degree　of　ionization.　At　cathode,　singly　charged　ions　are　dominant.　From　cathode　to　anode,　ions　are　ionized　so　that　doubly　particles　become　a　major　component　at　anode.　Finally,　atom　calculation　enables　the　accomplishment　to　combine　atom　with　ion　behavior　together.　Distributions　of　species　are　available　by　species　transport　equations,　providing　the　fraction　of　species.　©　2018　IEEE.