An ultra-wideband (UWB) band-stop filter (BSF) has been designed with flat responses at a selected frequency by cascading the unit electromagnetic bandgap (EBG) structures in this paper. The BSFs fabricated by the surface micromachining process have immense potential to compactness and low cost. The BSFs are constructed with comb-like EBG structures on a coplanar-waveguide (CPW) and have been investigated numerically. The simulation result of the BSF is demonstrated to have a good performance on its work band. It reveals a 20 dB stopband with a bandwidth of 6.8GHz. The pass-band insertion loss is less than 1.7dB.