A temperature-insensitive fiber Bragg grating (FBG) inscribed in ethanol-filled photonic crystal fibers (PCF) is designed and optimized. A structural PCF is proposed in which the diameters of the air holes of the first ring are larger than the other air holes. The structural PCF is designed and optimized via full-vector finite element method and genetic algorithms. The calculated Bragg wavelength shift (BWS) of FBG inscribed in ethanol-filled PCF is 31 pm while the temperature changes from −15  °  C to 55°C. When the temperature changes from 0°C to 40°C, the BWS is as small as 10 pm, and the thermal sensitivity of the Bragg wavelength is 0.25  pm  /    °  C.