The electron cyclotron maser (ECM) is a mechanism of electromagnetic radiation which is used to produce high-power and broad-band electromagnetic waves. It's a mechanism based on a stimulated cyclotron emission process involving relativistic electrons in a gyrational motion. The fi elds in the RF structure are used to extract energy from the relativitistic electrons in order to enhance their own intensity. Generally speaking, the RF structure may exist in the form of a resonant open cavity, a open-end waveguide, or a complex periodic structure. The main purpose of this thesis is to discuss, in view of physics, the phenomena produced by electron cyclotron maser in an ideal cavity.