Abstract: The differential pressure between mud pressure at bottom hole and pore pressure in formation influences the rate of penetration(ROP) when drilling oil and gas wells; but its mechanism is not clear. The pore pressure near the well bore bottom is not the original pore pressure; and it is related with location and time. This paper uses one dimension unstable seep model to describe and calculate the law of actual pore pressure under various times and locations. The shear stress and normal stress at any plan can be calculated based on crustal stress, bottom pressure, pore pressure and the stress caused by insert invasion. Using the Mohr-Coulomb model to judge the rock failure and get the invasion depth expression. Because the invasion depth of bit tooth is closed with ROP; with that expression the effect mechanism of differential pressure on ROP is studied and explained explicitly. The mechanism can be applied to some fields, such as getting higher ROP in under-balance drilling and providing an explicit parameter to calculate the suitable condition for dc index method

Key words: differential pressure, rock failure, pore pressure, under-balance drilling, dc index method