For measuring the planar microstrip patch antenna in millimeter-wave frequency range, the transition is usually required to connect to the waveguide interface of frequency extender then a vector network analyzer. The conventional solution is a two-stages transition by a waveguide-to-coaxial transition as a first stage and an end-launch adaptor as the second. Some works had proposed a directly transition from a waveguide interface to a planar microstrip interface. But the transitions of in-line structure will easily scattering that will affect the measured results, and the vertical structure will encounter the trade-off from the size of back cavity and the bandwidth performance. In this paper, a vertical type, waveguide-to-microstrip (W2ML) transition structure is proposed by the adoption of an aperture-stacked microstrip patch antenna (ASMPA) to have the performance of ultra-wide bandwidth and miniaturize the size. In addition, the characteristic of ASMPA W2ML transition can easily been by one-kit de-embedding method that only THRU calibration kit with time gating process is required. The consistency of the simulated result compared to the measured-calculated result exhibit the effectiveness of the proposed ASMPA W2ML transition for measuring a real planar microstrip patch antenna in millimeter-wave band.