Abstract: Leakage pressure is a main factor to induce formation leakage so that it can be utilized as a crucial parameter to analyze the causation of well leakage accidents,and that in-depth investigation on leakage pressure is of vital importance for secure drilling. By analyzing the characteristics of leakage formation,this paper divides the leakage into natural leakage and fractured leakage, and then defines the conception of minimal leakage pressure. Based on these work,the leakage mechanism of fractured formation is investigated,as well as the relationship between the leakage pressure,pore pressure and fracture pressure. Investigation results show that the currently existent prediction method of fracture pressure is established on the foundations of non-filtration borehole wall assumption as well as the Terzaghi effective stress model. These foundations are not consistent with the practical features of thief formation,which would inevitably cause deviation of calculated results with actualization. Besides,natural leakage formation constitutes the majority of formation leakage phenomena. Therefore, it is urgent to build up the leakage pressure curve instead of fractured pressure curve and take precautions against natural leakage.