Polarity has been considered as one of the important properties of an ionic liquid, which influences its design and industrial application. Though many empirical polarity scales of molecular solvents have been used to predict the polarities of ionic liquids, they always have their certain limitations. In the present work, ferrocene was selected as the cyclic voltammetry probe to investigate the polarities of ionic liquids. The experimental results suggested that the half-wave potential values of ferrocene/ferrocenium measured by cyclic voltammetry had a linear relationship with the E T (30) values of aprotic ionic liquids. Meanwhile, some polarity rules obtained here were in line with those summarized by other research studies. In addition, it was concluded that the polarity of a protic ionic liquid was slightly weaker than that of its corresponding Brønsted base. This work could provide a new and simple method to research the polarities of ionic liquids.