Abstract-A compact composite ultra wide-band elliptic-function low-pass filter is introduced by combining in cascade a microstrip stepped-impedance resonator using interdigital capacitor and an admittance inverter. A triple cascade low-pass filter is designed, analyzed and tested with this technique accompanied by its equivalent circuit model. This composition acts similar to a composite filter with desired attenuation and matching properties in order to obtain wide bandwidth and broad stop-band. The proposed low-pass filter features the sharpness of cut off frequency, low insertion loss, enhancement of bandwidth up to X-band frequencies and very compact size.Corresponding author: M. Nosrati (nosrati 1359@yahoo.com). 88Nosrati, Faraji, and Atlasbaf