In this paper, electromagnetic analysis of the reltron, which is a compact, simple, and efficient high power microwave source has been presented. The beam wave interaction process of the reltron oscillator has been analyzed to understand the device physics. The split cavity oscillator and relativistic klystron principles have been extended to demonstrate the electric field responsible for beam bunching and the electron beam modulation process in the reltron. The analytical formulation to obtain the RF energy growth and efficiency of the device has also been presented. To validate the analytical results and to evaluate the overall performance of the device, beam present simulation of reltron has been performed using commercial 3D PIC simulation code “CST Particle Studio.” With the parameters of a previously reported experimental reltron device, the present analytical calculation provided ∼240 MW RF output power with ∼38% efficiency while the PIC simulation provided RF output power of ∼225 MW with ∼36% efficiency at 2.75 GHz frequency. The obtained analytical and simulation results have also been found to be in agreement of ∼6% with this experimental work.