One of the main factors that affect transformer insulation aging is the core temperature. The higher the core's temperature is, the faster the insulation degrades and the shorter the transformer's life is. a design of sensor using optical Fiber Bragg Grating (FBG) was introduced to measure the transformer core temperature, which takes advantage of the slot on the core of the transformer to install the optical fiber Bragg ceramic casing grating temperature sensor on the core in column yoke surface of the transformer. When the temperature of core changes, the temperature of the optical fiber Bragg grating will vary with the core changes, which make the central wavelength of optical fiber Bragg grating shift, so the temperature of the core can be measured. When the running voltage is 75 V and 100 V respectively, the constant load experiments show that, under the constant voltage, when the ambient temperature is 15℃, in 0 ~ 105 minutes, the center wavelength of optical fiber Bragg grating stabilizes at 1 552.289 nm and 1 552.299 nm respectively, and the temperature rise are 17℃ and 18℃ respectively.