Refractive indices of several kinds of liquid crystals in the telecommunication band were measured by guided modes.We present the dispersion curve and corresponding coefficients of Sellmeier equations for the typical liquid crystal(5CB) from visible range ot infrared were presented.The principle,set-up and experimental data about such a measurement are given and discussed in detail.Experimental curve agrees well with the theoretical one which shows that the measurement is relatively accurate.Further,relation between the birefringence and molecular structure of liquid crystals in the telecommunication band was discussed.