The polychlorotrifluoroetlylene oil index matching fluids were studied and their performances, such as refractive index, absorption, isolation, breaking load and stability measured. The results showed that they can match KD*P crystals much finer than other kinds of matching fluids. The ref ractivity at crystal interfaces was improved by over 20 times and transmissivity increased over 10%. The fluid can be used to match calcite prism and good results were obtained.