A millimetre-wave test circuit of a transition from microstrip to waveguide using a four layer LTCC (Low Temperature Co-fired Ceramic) circuit board is presented. The transition is based on an aperture fed microstrip patch antenna radiating into the waveguide. The patch antenna is fed by a microstrip line, both integrated in the LTCC circuit. The waveguide interface is extended into the circuit using a via-hole fence. By this approach several transitions can be realized simultaneously on one LTCC circuit and allow free placement of one or more MMICs with bias circuits. The loss typically encountered going into a separate waveguide probe substrate is thus avoided. The design, simulation and test for a LTCC-based transition at W-band (75-110GHz) are described.