The eigen function of a bifrequency magnetically insulated transmission line oscillator （BFMILO） is deduced, and the dispersion characteristics and the field distribution are analyzed in detail. By dividing a traditional symmetrical coaxial disk loaded slow wave structures （SWS） into two different azimuthal partitions, the SWS of BFMILO becomes a two-dimensional SWS. In this structure, different electromagnetic modes are distributed in different azimuthal partitions, so that two different electromagnetic modes, which are competition modes in a traditional MILO, can interact with electrons separately and stably. This special SWS is the key point in designing a BFMILO. The proposed method can also be used in analyzing a mlti-frequency MILO.