Abstract: The parameters of higher order transverse magnetic modes TMn10 in coaxial cavity in high frequency band are investigated. It is found for given frequency that the cavity cross-section size and the TMn10 order can be freely chosen according to the power and purpose of the device. This implies that, even in high frequency band, large cavity cross-section size can be adopted, which avails to increase power. To widen bandwidth by making the working mode far away from neighboring modes and obtain great characteristic impedance, we can adjust cavity size and select mode according to the mode charts plotted. The calculation shows good agreement with the simulation using the 3D high frequency electromagnetic field software ISFEL.