Inductive Output Tube (IOT) is a vacuum electron tube capable of amplifying RF power with very good efficiency. IOT has been extensively used in TV services as UHF transmitters to amplify both audio and video signals. Its moderate power gain, high efficiency and long life make it suitable for this application. Besides that, this tube has been proving its capability and reliability in scientific areas like in high energy particle accelerators and fusion plasma heating purposes. The paper will present the RF design of input cavity of a low frequency (∼ 350 MHz) IOT carried out at CSIR-CEERI. It is an important component of the device. The simulation of the cavity has been carried out in CST and MAGIC2D codes. The CAD drawings have been prepared and the cavity structure has been fabricated. The details of design approach using CAD tools shall be presented in this paper.