Multi-agent　system　has　been　known　to　provide　distributed　and　collaborate　problems　solving　environment.　Each　agent　can　work　with　knowledge　base　to　improve　the　properties　of　reliability　and　real　time　of　system.　Currently　the　main　description　methods　of　Agents　are　formalized　languages,　but　the　description　capability　of　these　languages　about　activity,　interaction　and　evolution　of　Agents　is　deficient　obviously.　This　paper　abstracts　the　attributes　and　performance　of　agents　by　using　algebra　theory,　puts　forward　the　new　concepts　of　innovative　operation,　cooperating　operation　and　condition　operation,　and　investigates　properties　and　relations　among　them.　It　proves　that　the　set　of　all　Agents　of　a　multi-agent　system　and　its　innovative　operation　and　condition　operation　form　a　semi-group　respectively,　the　set　and　its　cooperation　form　a　commutative　semi-group.　The　operation　theory　of　Agents　proposed　in　this　paper　provides　a　new　method　of　Agent　description　and　establishes　theoretic　basis　of　more　researches　on　Agents　and　provides　theoretic　guidance　for　multi-agent　system　development.