On-line measurement of fiber core-temperature is very important for the investigations of high-power fiber lasers. Using thermal infrared imager is currently the most common approach to detect the temperature of fiber, even though it only monitor the surface temperature. So the measurement result is greatly different from the actual temperature of fiber core. Fiber Bragg Gratings (FBGs) have been widely used in detecting many physical quantities, such as strain, temperature, compression strength and curvature due to its intrinsic advantages, including simple manufacture technology, low insertion loss. Here, a new method based on FBG for measuring the temperature of fiber core is presented and experimentally demonstrated. We use a mutiple inscription method that inscribe a low reflectivity FBG on the high reflectivity chirp grating location as a fiber core temperature on-line monitor. Due to the resonant wavelength of chirp grating (1080nm) is far away from FBG’s resonance peak (1550nm), it means that the presence of FBG will not impact the performance of the chirp grating. Alongside this composite optical fiber structure is applied to fiber oscillator we inject a amplified spontaneous emission (ASE) light with the wavelength range near 1550 nm from the signal fiber of (2+1)*1 combiner. And then, the reflection spectrum of FBG can be record from the port 2 of circulator. The temperature of chirp grating and low reflectivity FBG can be measure by recording the shift of FBG’s resonance peak after the lineal relation between resonance wavelength and temperature are cofirmed. In order to detect the accuracy of this new structure, we also use Optical Frequency Domain Reflectometer (OFDR) to measure the temperature of the chirp grating. Experiment results prove the high accuracy of proposed temperature monitor. It is strongly believed that the novel proposed structure can be used to achieve measurement of fiber core temperature in high-power fiber lasers. This work also provides a novel idea for manufacturing multi-functional composite fiber structure.