We report a frequency tunable scheme for the gyrotron at millimeter/submillimeter regime. Unlike the step-tunable type where oscillation frequencies change discretely, this scheme continuously adjusts the oscillation frequency as the magnetic field varies. It is a hybrid system, taking the advantages of the backward-wave interaction and the converter-free output structure. The characteristics of backward-wave interaction will be shown. A proof of principle experiment was conducted with a scaled cavity. The result shows the oscillation frequency smoothly transitions over a wide range of 6 GHz from 134 to 140 GHz. With proper design this mechanism is capable of producing medium output power with broad frequency tunability up to the terahertz region.