Abstract: Literatures always introduce the relationships between different fault structures and principal stresses. But it was not emphasized that these relationships exist only before occurrence of these faults. No literature introduces the relationships between different fault structures and principal stresses after occurrence of these faults. It makes people always think that the directions of principal stresses keep unchanging after occurrence of the faults. In the area of normal and reversed faults,about thousand tests of in-situ stress measurements by hydrofracture in oil wells were conducted in 86 oil fault fields of 13 big oil regions in China. The results of stress measurements indicate that the directions of cracks by hydrofracture are basically perpendicular to fault strike in normal and reversed fault area,which means that the minor principal stress is always horizontal stress parallel to fault strike. This conclusion is different from that of existing literatures. The methods to calculate the in-situ stresses after fault occurrence are introduced,and the above questions are answered quite well.