A novel higher order mode depressed L-band magnetically insulated transmission line oscillator (MILO) device is put forward by destroying the-mode oscillating condition of HEM11 mode. High frequency analysis and PIC simulation are carried out. The simulation results reveal that the novel MILO device oscillates in TM00 mode and successfully suppresses the higher order HEM11 mode generation. Contrastive experiments are carried out. Purified 1.3 GHz microwave obtains a corresponding fundamental mode oscillation in higher order depressed MILO in contrast with original MILO which obtains a 1.42 GHz microwave output.