Aiming for electron cyclotron heating application in plasma fusion system, a 28 GHz 50 kW gyrotron has been developed and fabricated. The gyrotron employs a triode magnetron injection gun, a conventional cylindrical cavity working under TE02 mode, a built-in quasi-optical mode converter with conversion efficiency of about 95%. In the experiment, Gaussian beams of 54.8 kW for 1 s pulse and 45.6 kW for 30 s continuous wave at 28.08 GHz were achieved. The corresponding total efficiency is 57%.