In this paper, we investigate a reference-plane invariant (RPI) method for electromagnetic property extraction of bi-axial bianisotropic metamaterial (MM) slabs. In order to obtain unique properties, we applied the frequency varying technique in order to determine the location of the slab within its cell. For validation of the proposed method, we first simulated and then measured scattering parameters of a MM slab constructed by split-ring-resonators, next extracted its electromagnetic properties, and finally compared extracted properties by those retrieved by another method which requires reference-plane information. From the comparison, we noted that whereas our proposed method correctly retrieved electromagnetic properties without knowing the value of reference plane transformation factors, the accuracy of the tested method was seriously influenced by any inaccurate reference plane information. (C) 2018 Elsevier B.V. All rights reserved.