RTIL consisting of 1-methyl-3-butyl-imidazolium(BMIm) cation and bis(trifluoromethane sulfone) imide(TFSI) anion was mixed with 1-methyl-3-n-butyl-imidazolium hexafluorophosphate(BMImPF6) at different ratios.Their room temperature conductivity,MS,LV were measured and the effect on association phenomenon by mixing is analyzed.The results demonstrate that the mixtures of ionic liquids can reduce the association degree of single ionic liquid with the better effect of properties,which mesans that the properties of the mixtures are not the average of single RTIL,but have a synergism effect.