Abstract: With the development of free electron lasers (FELs), high brightness microwave electron gun is being paid more and more attention. An thermionic cathode microwave electron gun with monocrystal LaB₆ was designed and tested in the Institute of High Energy Physics, in order to meet the requirement of Beijing Free Electron Laser (BFEL).This paper presents the methods of solving the problems encountered in the research such as surface treatment, holding and heating of LaB₆. The electron beam melting was used to melt LaB₆ with base metal tantalum at first. The emission current density is 84A/cm², the life time of the cathode is 200 hours and the lifetime of the heater is longer than 500 hours.