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Summary form only given. 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.