A new two‐step calibration technique to directly measure the scattering parameters of asymmetrical transitional structures is proposed in this article. This technique conducts two calibrations, each one of them for one type of the transition's two ports, and two error boxes associated with the transition measurement system can then be obtained, with which the transition's raw measured data can be corrected. Experimental studies have verified the validity of the formulation and the accuracy of the two‐step calibration technique. The technique can be extensively used to measure the scattering parameters of transitional structures without resorting to constructing back‐to‐back test fixtures. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 132–135, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10846