A novel microstrip diplexer with high isolation is proposed. The diplexer is comprised of two novel cascaded quadruplet bandpass filters. The cascaded quadruplet section consists of two half-wavelength resonators and two quarter-wavelength resonators, so the circuit won't occupy much space. By introducing multipath coupling, numerous transmission zeros are generated on each side of the passband and high isolation is achieved. Based on the proposed structure, a diplexer with lower and higher channels centered at 2.0 GHz and 2.4 GHz, respectively, is designed and fabricated. The measured isolation between the two channels is larger than 47.3 dB.