An universal relationship between a refractive index of a premixed gas of fuel and air and its equivalence ratio has been discoverd in this study. Using this relationship, we can calculate the equivalence ratio of all the premixed gas from its refractive index only with a unique coefficient, exept for the premixed gas of light fuels. The uniquness of the relation coefficient enables us to obtain the equivalence ratio of even multi-fuel premixed gas from its refractive index. In this paper, the novel relationship is applied to a non-scaning 3D-CT(Computer Tomography) technique using a multi(20)-directional quantitative schlieren system with flash light source. Instantaneous equivalence ratio distribution of a bi-fuel premixed gas flow of two jets of propane/air and gasoline/air mixtures is measured by the CT reconstruction.