Abstract: Free Electron Lasers（FELs）require very good quality electron beam source.A lowemittance is necessary to obtain short wavelength A high peak current provides a high gain。Ahigh-gradient,S-band 1 + 1/2 cell microwave gun（i.e.RF gun）with a LaB₆ thermionic is being developed in CAEP. The design principle,fabrication and testing of the RF gun are described.TheRF gun had been first tested and produced electron beam in Dec,1992.After one year of thetesting and operating,beam emittance and electron energy spread are also measured.These resultsare very satisfactory.When input microwave power in to the RF gun is about 1.1 MW,the mainparameters of the RF gun are following:maximum electron energy is 1MeV,average electronenergy is about 0.6MeV, routine macropulse current is 1A,maximum macropulse current is 1.67A,macropulse-width is 2μs microwave frequency is 2997.75MHz, repetition rate is 50Hz,electron beamnormalized emittance is 5 π mm·mrad.