A new octave bandwidth high selectivity compact microstrip diplexer working in the band from 10 to 20 GHz is presented, intended to be a part of an electro-optical interferometer covering the whole frequency band. The circuit is based on the combination of hybrid couplers and bandpass filters both on the microstrip technology. The diplexer provides two output frequency bands which cover the 10-14 GHz (relative bandwidth of 33%) and the 16-20 GHz (relative bandwidth of 22%) ranges, respectively, with a stop band in between from 14 to 16 GHz. Measured results show a insertion loss level of 3.3 dB for both output bands and high selectivity performance, exhibiting a rejection level between output bands higher than 20 dB and return loss better than 10 dB in all ports.