A novel nanocomposite Mo-4%La2O3 cathode has been prepared by sol-gel method and hydrogen reduction process and then hotpressing sintering technique. The lanthana particles in the nanocomposite cathode were less than 100nm, and they were about 300-400nm in the coarse-grained cathode. The particle size of lanthana has a strong effect on the field emission characteristics of cathode. The nanocomposite cathode has superior field electron emission ability than that of the coarse-grained 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 coarse-grained cathode concentrate on a small area of the cathode surface. The reasons for the different distribution of emission sites were discussed.