While oil shale is heated to a certain temperature, its porous structure undergoes continuous modification and as a result, the rock permeability will also change with increasing temperature. In this paper, the mercury injection apparatus was used for measuring the pore size of oil shale samples at different temperatures. The results showed that, firstly, as the temperature increased, the total pore volume, average pore size and porosity of oil shale increased significantly. When the temperature reached 600 degrees C, the porosity was increased to 34.6%, making 8.3 times the initial porosity. Secondly, during the heating of oil shale, the volume of mesopores (0.1 mu m < d <= 1 mu m) increased continuously while that of micropores (d <= 0.015 mu m) decreased. Because the solid organic matter in oil shale was subjected to pyrolysis, micropores began to merge to become medium-sized or mesopores, or even macropores. In the temperature range of 400-500 degrees C, organic matter was pyrolyzed heavily and both the increment of medium-sized pores percentage and the decline of micropores percentage were very significant. Thirdly, the oil shale permeability increased with rising temperature, being at 600 degrees C 3.0 x 10(-8) m(2), which is nearly 600 times that at the room temperature stage of pyrolysis. Shortly, heating oil shale to high temperature can generate new pores and merge the existing ones, which will prompt transforming the tight oil shale into porous rock with high permeability and favour oil and gas seepage.