Abstract: For miniaturization and harmonic suppression, a novel planar compact lowpass filter is presented in this paper. A conventional stepped-impedance lowpass filter is modified by some microstrip steps connected in 90° and some T-shaped transmission lines integrated into the lowpass filter. The proposed lowpass filter has the advantage of low insertion loss (below 0.5 dB at 0～4 GHz), ultra broad stopband (over 40 dB at 5～30 GHz) and compact size (50% reduction). Experiment validates the design concept.