This paper presents a millimeter-wave microstrip-based sub-harmonic mixer with a wide operation band. In this design， frequency suppression circuits including a wideband bandpass short-circuited filter and a diplexer are employed not only to provide proper terminations for the intermediate frequency （IF）， radio frequency （RF）， and local oscillator （LO） signals simultaneously， but also to reject the major idle mixing products. The measured results show that the proposed sub-harmonic mixer can support the operations in RF band from 27 to 48 GHz， and in IF band up to 6 GHz. Meanwhile， the conversion loss is less than 12.5 dB for both up- and down- conversion throughout the bandwidth， in which， the minimum conversion loss is about 7.5 dB and 8.2 dB for the down-conversion and up-conversion， respectively， at an RF of 33 GHz and IF of 1 GHz.