In this paper, a 3-D electromagnetic (EM) and particle simulation of an X-band high power gyrotwystron has been reported. The EM (cold) and the electron beam and RF wave interaction (hot) behavior have been studied using a commercially available 3-D particle-in-cell code MAGIC. The EM field analysis guarantees that the RF circuit operation is in the desired TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> at the input cavity and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> circular electric waveguide mode at the output with the desired frequency of operation of 9.8 GHz. The hot simulation demonstrates the particles phase space behavior along the interaction circuit and the energy transfer phenomena. The present gyrotwystron develops an output power ~20 MW with an electronic efficiency of ~21% and a bandwidth of 10% for ideal electron beam velocity.