Applications of fibre optic sensors at high temperatures have gained a huge interest recently, as they appeared to be suitable for temperature recording in harsh environments. In this paper, we are demonstrating two intrinsic Fabry-Perot ( F-P) fibre optic sensors for high temperature monitoring. The sensors are consisting of a 125 mu m diameter single mode fibre ( SMF28) and a 125 mu m diameter PCF ESM-12B pure fused silica fibre spliced to a SMF28, respectively. The result was a low finesse optical SMF-Cr-SMF, and SMF-Cr-PCF, sensor with cavity lengths varying from 50 mu m to 100 mu m. Both types of Fabry-Perot sensors were tested in a tube furnace over a temperature range from room temperature up to 1100 degrees C. Following a number of annealing cycles, between the above mentioned temperatures range, very good repeatability of the phase response was achieved. During the cycling process, thermal stress relief takes place which makes the sensors suitable for temperature testing at temperatures just in excess of 1000 degrees C. After initial cycling the sensors are subjected to long term stability tests. The phase response is stable, less than 4 degrees C, over a period of 5 days at a temperature of 1050 degrees C for both sensors. The temperature resolution is around 3 degrees C.