A very simple method of experimental determination of complex permittivity or complex permeability of single-negative (SNG) metamaterials in waveguide environment is proposed in this paper. The method uses only one measurement and it is based on the fact that any SNG material can be thought of an equivalent evanescent transmission line. The input impedance of this line is equal to the wave impedance, almost regardless of the line termination. The method was tested by measurements of complex parameters of SRR-based mu-negative (MNG) metamaterial and thin-wire-based epsilon-negative (ENG) metamaterial operating in 10 GHz frequency band and the results were found to be in good agreement with theoretical predictions