A　nanocomposite　Mo-4wt%　La2O3　cathode　was　prepared　by　a　high-energy　ball-milling　and　hot　pressing　technique.　The　sizes　of　lanthana　particles　in　the　nanocomposite　La2O3　cathode　are　less　than　100　nm;　in　contrast,　the　sizes　of　thoria　particles　are　about　1　similar　to　2　mu　m　in　a　commercial　W-4wt%　ThO2　cathode.　The　average　vacuum　arc-starting　field　intensity　of　the　nanocomposite　Mo-La2O3　cathode　is　2.97　X　10(7)　Vim,　which　is　62.7%　lower　than　that　of　the　commercial　W-ThO2.　The　nanocomposite　Mo-La2O3　cathode　exhibits　superior　electron　emission　performances,　and　its　distribution　area　and　thickness,　of　electron　emission　spots　are　remarkably　larger,　as　compared　to　those　of　the　commercial　W-ThO2　cathode.　The　size　of　oxide　particles　has　a　great　effect　on　the　electron　emission　performances　and　vacuum　arc　characteristics　of　cathode.　The　electron　emission　performance　of　Mo-La2O3　cathode　will　be　improved　with　decreasing　of　the　lanthana　particles　size.　When　La2O3　particle　size　decreases　to　less　than　100　nm,　the　electron　emission　area　and　ability　of　the　Mo-La2O3　cathode　significantly　increase.　The　much　enhanced　electron　emission　performance　of　the　nanocomposite　Mo-La2O3　cathode　is　attributed　to　the　formation　of　a　higher　inter　electric　field　and　space-charge　regions　at　the　interphase　boundaries　between　Mo　and　La2O3　phases.