In this paper, we report on the development of a microstrip-to-waveguide transition for the WR-1.5 waveguide band (500–750 GHz). The microstrip lines and E-plane probes have been manufactured on 25 μm thick GaAs substrates. The transmission loss per single microstrip-to-waveguide transition is only 1.0 dB @ 670 GHz. The measured return losses are better than 10 dB up to 720 GHz. The single transition includes a waveguide section with a length of 7.0 mm corresponding to the transitions which will be used in future submillimeter-wave MMIC modules.