Thermo-optic effect of liquid materials at optical wavelength of 1550 nm is investigated by use of a side polished fiber Bragg grating (FBG) sensor with double reflective Bragg wavelengths. The relationship between the difference of double reflective Bragg wavelengths of sensor and refractive index of overlaid liquid material is measured, and the relationship between the difference of double reflective Bragg wavelengths and temperature is measured when a liquid material overlays on the polished region of sensor. By analysis of the relationships above the thermo-optic coefficients of three liquids made by Cargille Lab are obtained at optical wavelength of 1550 nm. The fiber sensor could be used for measurement of thermo-optic characteristic of liquid materials as their refractive indexes are in the range of 1.4198～1.4479.