Abstract: A mixed-mode through-reflect-line(TRL) calibration algorithm based on symmetrical reflection standard was proposed to accurately characterize coupled differential devices using scattering parameters. Using the transition structure’s generalized reciprocity and the phase relationship between its scattering matrix entries, the sign ambiguity was removed and the transition structure’s mixed mode wave transfer matrix was obtained. The calibration standards of the proposed algorithm are similar to the requirement of traditional single-mode TRL algorithm, and unsymmetrical structured reflection standard is not necessary to yield mode conversion. To verify the proposed algorithm, the single-mode scattering matrix of calibration standards and device-under-test(DUT), both of which are embedded with the transition structures, were calculated from full-wave electromagnetic simulation, and the proposed algorithm was used to process the simulation data to calculate the DUT’s mixed-mode scattering matrix. The calibration results were compared with that from full-wave simulation, and the agreement of the data obtained through both approaches indicates that the proposed algorithm is valid to calibrate linear differential devices.