Abstract: The reservoir fracture and microfracture formation mechanism under ground stress has been stated in this paper. Through the application of XMACⅡlogging technique, the effect of the maximum principal stress orientation in horizontal direction on water flood development in oil reservoirs has been analyzed.The fracture orientation and the maximum ground stress are both affected by water flood.When the fracturing effect of water injection well is ideal,oil production along the maximum principle stress orientation could reach the effect of liquid and oil increment,while if not, good results are difficult to be achieved.