A microstrip quadruplexer with a wide stopband is presented in this paper. The quadruplexer consists of asymmetric stepped-impedance resonators (SIRs) and a distributed common feeding line. In order to increase the stopband bandwidth, the higher order resonant frequencies of the asymmetric SIRs are staggered by properly selecting impedance ratio K and length ratio. Thus, a wide upper stopband performance can be achieved. The first, second, third, and forth channels of the quadruplexer are designed at 1.5, 1.8, 2.1, and 2.4 GHz (denoted by f1, f2, f3, and f4). For the upper stopband performance, the rejection level is greater than 20 dB up to 11.4 f1 (17.1 GHz).