BARC has initiated a program on the indigenous development a Klystron that can deliver 6 MW (peak)/24 kW (average) power. Ceramic window is one of the vital sub-systems that need special attention due to its susceptibility to RF, thermal and multipacting breakdown. The paper describes the design of ceramic window. It consists of a cylindrical waveguide with Alumina disk operating in TE11 mode that is terminated in a std. WR284 rectangular waveguide. Window parameters like ceramic material, disk thickness, diameter, cylindrical waveguide length, diameter and length of WR284 waveguide is optimized using CST Microwave studio. Alumina material is used as ceramic due to its high dielectric constant (k = 9.9) and low loss tangent (tan δ ~ 0.0001). A maximum bandwidth of ~8 MHz is obtained at central frequency of 2857 MHz at a return loss of 21 dB after optimization. Maximum tangential and normal electric field of 463 V/m and 328 V/m is observed in the center of the Alumina disk. Maximum magnetic field of 1.1 A/m is observed at a radius of 32.86 mm. Maximum power loss density in the overall structure is ~16.86 W/m3. Thermal analysis is carried out using ANSYS. A cooling jacket is designed; water flow rate is estimated. Multipacting analysis is carried out using CST particle studio at different power levels at different initial positions of the primary electron. A transition is designed between output cavity of klystron and the RF window to minimize reflection losses.