A lowpass filter with a very sharp transition band using defected ground structures (DGS) is described. The lowpass filter was designed by using three dumbbell slots at the ground plane. Two of those dumbbell slots are the reshaped rectangular and the third dumbbell slot is an inverted triangle. The lowpass filter designed at cutoff frequency of 3.0GHz, which is suitable for GSM900, GSM1800 and UMTS applications of the mobile communications. The equivalent circuits for the proposed lowpass filter and its corresponding LC parameters are given. The proposed lowpass filter provides a size of 33 × 30mm2, and have good transition band with good performance in the passband and have wide rejection up to 9.25GHz plus harmonic suppression in the stopband. Measurements results show good agreements with the simulated results.