Presented in this paper are two methods used for determining the critical field strengths in electronegative gas mixtures. One of the two methods is to use the predischarge parameters of the gases for the determination of the mixtures with any concentrations. And the other is to use the recordings of the electron current waveforms in the gas mixtures studied in a discharge chamber. The critical strengths in mixtures of SF6 and N2 were determined by the two methods and good agreement has been achieved.