Abstract: Hydraulic fracture of coal seam roof is one of the important means to deal with hard roof and prevent rockburst. In order to study the variation of three dimensional stress in rock owing to hydraulic fracture of coal seam roof, a type of Fiber Bragg grating (FBG) sensor was adopted for monitoring the stress in rock before and after hydraulic fracture, through the field test of hydraulic fracture of coal seam roof. The results show that the magnitude of three principle stress decreases after hydraulic fracture, and the biggest drop is about 20%; meanwhile the direction of three principle stress changes very little. In the working surface coordinate system, the decline of every stress component is affected by the orientation of the fracture plane. Hydraulic fracture of coal seam roof is able to significantly reduce three dimensional stress in roof rock, which has good effect on pressure relief. This study can be used as reference for the layout of hydraulic fracture boreholes, and for the prevention and control of rockburst.