Abstract: This paper describes the design of the RF circuit and gun for an X-band high power CW klystron. The RF circuit consists of six cavities. One-dimensional large signal simulation results indicate that the circuit has an efficiency of more than 50% with a saturated gain of 50dB. The gun has a perveance of 1.0μp. The beam focusing system is a solenoid magnet structure. Simulation indicates that the beam-to-tunnel radial fill factor less than 0.6, and the beam ripple less than 7%.