Abstract: In this paper, the effects of relative directions of principal stress and the fault trend and their variation on the preparation of earthquakes have been discussed along with stress direction measured by the hydrofracturing. It is obtained that the viscous slip may occur on the surfaces between the active faults under the condition of the stress far less than the seismic stress; and the relative slip at different direction may result in various effects. The reliability of the judgment for a region stability from the relative direction of principal stress should be very high.