A sharp-rejection quad-band microstrip bandpass filter with five transmission zeros is proposed. The bandpass filter consists of a quad-mode resonator and two triple-line feeding lines. Analysis on how to control the passband frequencies and the transmission zero frequencies is given. The four passbands are at 1.6 / 2.3 / 3.1 / 5 GHz with the insertion loss of less than 1.2 / 2.2 / 1.2 / 2.3 dB and the return loss of greater than 22 / 14 / 21 / 16 dB. Furthermore, the five transmission zeros are evenly distributed beside the passbands at 1.3, 2.1, 2.7, 4.1, and 6.1 GHz.