It is well known that the dielectric strength of ternary gas mixtures comprised of N₂, SF₆, and small amount of c-C₄F₈ is much higher than that of binary gas mixtures of N₂ and SF₆ under both of uniform and non-uniform DC fields. Electron swarm parameters of the binary and ternary gas mixtures are calculated by a Boltzmann equation. Especially, the authors focus ionization coefficients and electron attachment coefficients of the gases, and analyze and discuss the dielectric characteristics to investigate basic physical processes with respect to the mixing of gases from a point of ionization and electron attachment phenomena. Consequently, it is shown that the improvement in breakdown strength of gases results from the fact that more electrons attach to SF₆ due to the effects of c-C₄F₈ which acts as increasing the number of electrons in low electron energy range under low mixture ratios of c-C₄F₈.