A compact microstrip lowpass filter (LPF) for harmonics suppression is proposed. An open stub LPF with uniquely shaped defect ground structure cells is used to optimize the LPF performance. A stopband attenuation of greater than 20 dB is obtained from 3.42 GHz to more than 18 GHz. The physical size of the proposed LPF, excluding transmission lines, is only 10 × 15 mm 2. Good agreements between simulation and measurements are observed.