Radiation from a ferrite-filled waveguide with multiple thick slits is investigated. The Fourier transform and mode matching are utilized to obtain a rigorous solution. The effect of the ferrite linewidth is included to account for the magnetic loss of ferrite. The maximum beam steering ratio 4.33&DEG;/100 oe is achieved with the magnetic bias field ranging from 1000 to 1400 oe. The ferrite-filled rectangular waveguide with nine slits is fabricated and its radiation patterns are measured at X-band. Our computational results agree with measurement data.