Abstract: In this paper, a novel commensurate line lowpass filter is presented and synthesized, which has equiripple in-band response and ultra-wide stopband. The initial filter topology is based on a “Z” shaped coupled-line section, on whose four nodes two pairs of open-circuited stubs are symmetrically loaded. All of those transmission line segments are identical in electrical length, as a result, the transfer function of the whole structure can be obtained after microwave network analysis. By using the equiripple matching procedure, the transfer function is forced to map the prescribed specifications, such as cut-off frequency and in-band ripple level. In the fabricated sample filter, radial stubs are used to replace the open-circuited stubs which have low impedance. The measured and simulated results agree well, which validate the design theory successfully.