In this paper, we have finished seepage experiments of natural cores from two different areas. And analyzing the data from experiments, we got the curves of permeability and threshold pressure gradients. We found these curves existed good correlation in logarithmic coordinates. We also found that their threshold pressure gradients are different at the same permeability. Seeking the reason why they were different, we checked the experimental process and also tested natural cores form these two areas with the technology of constant speed mercury injection. We discovered that their micro pore structure of cores are different, which can explain the difference of the threshold pressure gradients. The smaller the pores and throats are, the greater the specific surface area is, the more greatly the fluid flowing were affected by boundary layer, the bigger the threshold pressure gradients are.