Travelling wave tubes (TWTs) are power amplifier for communication satellite. As the electron gun plays a significant role in determining the life and reliability of the space TWT, the same has been subjected for 3-dimensional thermal and structural analysis using FEA code COSMOS and ANSYS. Expansion in different electrodes like cathode, beam focus electrode (BFE) and anode in axial and radial direction has been simulated due to temperature distribution in the gun. In addition to the above, frequency analysis of electron gun assembly has also been carried out. A comparison of COSMOS and ANSYS results has been presented.