The pores in oil shale, which act as channels for the migration of products of cracking of organic matter and the place for heat transfer in the rock mass, directly influence pyrolysis efficiency. In this paper, the pore characteristics of oil shale during pyrolysis under the convection and conduction modes of heating were determined by mercury intrusion porosimetry (MIP). Results show that in case of both the heating modes, the threshold temperatures for transformation of pore structure from simple to complex are 382 degrees C and 452 degrees C, respectively. The porosity of oil shale subjected to convection heating is generally higher than that subjected to conduction heating. By the convection heating mode, the high-temperature fluid can extract the shale oil attached to the pore wall and increase the porosity. As the pyrolysis temperature increases from 314 degrees C to 555 degrees C, the average pore size of oil shale increases from 23.70 to 218.15 nm in convection heating and from 21.68 to 145.60 nm in conduction heating. During the pyrolysis of organic matter and extraction of oil and gas, high-temperature steam continuously widens the pores. Finally, when the pyrolysis temperature is above 314 degrees C, pores with a smaller size gradually change into mesopores and macropores with a larger size. It is proved that under the convection heating mode, oil shale changes from a dense rock to a porous medium with an obviously higher amount of pores.