Lanthanum-containing　cathodes　are　an　attractive　alternative　to　thorium　tungsten　cathodes　because　of　their　low　work　function　and　their　comparatively　low　operating　temperatures.　The　function　of　La　element　in　La-Mo　cathode　material　was　studied　by　using　a　set　of　equipment　specially　designed　to　cathode　research.　A　La-Mo　film　cathode　was　prepared　with　pulsed　laser　deposition　(PLD)　in　diverse　vacuum　condition.　For　the　first　time,　the　electron　emission　properties　have　been　measured　without　exposing　to　the　atmosphere.　By　means　of　Auger　electron　spectroscopy,　we　investigated　the　surface　composition　of　the　cathode　in　situ.　We　find　out　obviously　that　the　cathode　emission　properties　have　relation　to　the　surface　La/O　ratio.　As　the　La/O　ratios　get　higher,　the　emission　properties　show　better.　The　excess　La　element　is　contributed　to　thermionic　emission.　On　the　basis　of　experiment　result,　the　emission　mechanism　of　La-Mo　cathode　has　been　discussed.　©2004　IEEE.