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Nanocomposite Mo-4La2O3 (mass fraction) cathode was prepared by sol-gel method and hydrogen reduction process and then hot-pressing sintering technique. The microstructure of nanocomposite cathode is homogeneous and very fine; the lanthana particles are about 40-50 nm, while they are about 200-300 nm in the conventional Mo-4La2O3 cathode. The particle size of lanthana has a strong effect on the field electron emission ability of cathode. The nanocomposite cathode has superior field electron emission ability compared with that of the conventional cathode. The sites of field electron emission of the nanocomposite cathode nearly extend over the whole surface of cathode, whereas the emission sites of the conventional cathode concentrate on a small area of the cathode surface. By creating the electron interaction model between the metal and semiconductor, the relation curves between the tunneling probability of electrons in Mo and La2O3 and second phase size were calculated and plotted, respectively. The reasons that nanocompposite cathode has stronger electron emission performance were explained.