In modern RF/microwave communication system, low-loss, lightweight and cheap band stop filter is highly demanded to attenuate undesired signal in a certain frequency range. To achieve reconfigurable subsystem in multi-band systems and radar systems, tunable RF/microwave band stop filter is necessary. In our work, a C-band tunable band stop filter device is designed, fabricated and tested. The device is designed with a single transmission line deposited on a narrow magnetic film. The magnetic film adopted is multi-layer FeGaB/Al2O3 film. Considering its relatively low ferromagnetic resonance linewidth, FeGaB material has great potential in RF/microwave filter device. Compared to single layer magnetic film, multi-layer film has lower eddy current loss and out-of-plane anisotropy. The device performs a self-biasing resonance at C-band by taking the advantage from the shape anisotropy of a long FeGaB/Al2O3 multilayer film. The self-resonance of the band stop filter appears at 5.5 GHz with an attenuation of over 20 dB. The device can also be magnetically tuned up to 9 GHz (64% tunability) with an attenuation of over 50dB by applying an external magnetic field of less than 400 Oe. The power handling capability under various magnetic biases is measured, where a maximum of 3.7 dBm IP1dB power capability is exhibited under zero bias field.