Although sulfur-hexafluoride (SF 6 ) has been used as an insulating medium in various power system applications it has been shown that its greenhouse effect is significant. Due to this effect throughout the world, many countries have already issued restrictions on the usage of SF 6 , which may adversely affect various apparatus and systems presently being used in power industry. In order to solve the problem without a delay studies have been undertaken to find a replacement gas for pure SF 6 so that the existing insulation of a power apparatus or a system can be modified. Among many alternatives, nitrogen and SF 6 (N 2 + SF 6 ) mixtures seem to be the simplest and the least expensive alternative due to N 2 being inert, abundant and renewable. Because SF 6 is an electronegative gas, the synergism may take place in mixtures of SF 6 with N 2 . The synergy in a gas mixture is considered when the value of the dielectric strength of the mixture is higher than the value determined by the linear combination of the dielectric strengths of each component gas in the mixture. Therefore, in this study the degree of synergism is assessed due to electronegativity of SF 6 and the non-linear behavior of this binary mixture in order to determine if there exists an optimum SF 6 concentration.