his manuscript presents the mode selection for 240 GHz, 1 MW gyrotron for futuristic plasma fusion machines. To minimize the Ohmic wall loading and space charge effect, several very high order TE modes (m>40, p>12) are analyzed in detail. The voltage depression, Ohmic wall loading, frequency separation from the nearest neighboring modes, etc., are calculated. An in-house developed computer code GCOMS is used for the mode selection process. Simple cylindrical cavity is considered in place of coaxial cavity for 240 GHz gyrotron due to easy fabrication.