A novel waveguide directional filter with compact-size and low insertion loss is proposed in this letter. The basic structure consists of a pair of rectangular waveguides placed parallel to each other along their narrow side and two identical band-pass filters connect them via two coupling slots. For easy of fabrication and assembling, all the band-pass filters and waveguides have a uniform height, that is all the components of the directional filter are in a single-layer waveguide. It is easy machining by standard computer numerical controlled (CNC) techniques to match the requirement for low cost. For verification purpose, an X-band single-layer waveguide directional filter has been manufactured and measured. The measured results show a insertion loss better than 0.1 dB, a return loss lower than −17.5 dB and a isolation better than −22.5 dB, good band-pass and band-elimination responses are also obtained over the frequency range from 7.25 GHz to 9.75 GHz. The good agreement has been found between the simulated and measured results.