Abstract: The linear dispersion relation of the electromagnetic waves in a plasmapartfilled slowwave structure is analyzed and calculated. Dispersion curves of the TM01 is up shift to 1GHz as the plasma density increases to 8×1011cm-3 with the plasma thickness of 0.2cm. The result is in good agreement with the experimental result observed by X·Zhai. It is found that a series of lowfrequency plasma modes satisfy Floquet theorem in the magnetized plasmapartfilled slowwave structure. These modes can interact with relativistic electron beam and the peakgrowth rates of some modes are as high as that of TM0n modes. Increasing the plasma density, the peakgrowth rate of the plasma mode increases quickly, and becomes greater than that of the TM0n modes. For the plasma density greater than