Coaxial disk-loaded waveguide has been widely used in magnetically insulated transmission line oscillators (MILO). Although TM00 mode is assumed to be its main mode, the asymmetric mode might be excited by slight asymmetry of the structure, asymmetric emission of the beam, or both. So the research on asymmetric mode in coaxial disk-loaded waveguide is necessary. The general eigen equation with both symmetric and asymmetric modes in coaxial disk-loaded slow-wave structures is theoretically analyzed. Then, a HEM11 mode working MILO is designed and a tentative experiment is carried out to test its performance. Employing an electron beam of about 480 kV and 39 kA, HEM11 mode working MILO generates a high power microwave output of about 1.2 GW with 40 ns pulse duration at 1.98GHz. The power conversion efficiency is about 6.5% and the generated microwave mode is TE11 in the coaxial output region.