Parasitic oscillations can have a high impact on high power Gyrotron operation.They can degrade the beam quality,reduce the interaction efficiency and reduce the power level.Furthermore,they can affect the stability of the gyrotron operation leading to unwanted oscillation modes which may cause serious damage to the gyrotron.In this paper,a study on parasitic oscillations in a 2s,118 GHz,0.5 MW X--‐3 gyrotron at CRPP is presented.A frequency counter(70--‐170 GHz)with~20 ms acquisition time was used to monitor the dominating mode frequency during the entire pulse.Two heterodyne systems were used to monitor the spectrum of the output RF in detail.One system is based on a dual--‐channel harmonic mixer arrangement with a fast oscilloscope which can record~6 GHz bandwidth spectrum with maximum time duration of 4 ms with unambiguous RF frequency identification.The other is based on a harmonic mixer and a 16 channel band--‐pass multiplex unit which can record pulsed and CW signals.Parasitic oscillations were recorded and compared during different parts of the pulse.The gyrotron operation parameters have been mapped to check the behavior and evolution of these parasitic oscillations.In this study it is found that certain parasitic modes can cause a significant increase of the cavity losses.