Abstract: It is confirmed that transparent cathode technology has a significant influence on oscillation start-up process in relativistic magnetron, while the fast start-up mechanism is under further evaluation. Electromagnetic field distribution and charged particle motion were simulated in an L-band relativistic magnetron with transparent cathode. Results show a great difference between solid cathode and transparent cathode. Both electrostatic field distribution and electron spoke are modulated by transparent cathode structure. To obtain an optimal operating state, transparent cathode and relativistic magnetron should be matched in a proper configuration.