The micron-scale (mu m) pore is the main channel of fluid percolation. The evolution of mu m-sized pore structures in a cylindrical oil shale sample (phi= 0.82 x 7 mm) at different temperatures was investigated by high-precision micro-computed tomography (micro-CT) scan technology. It is found that there is a small change in pore structure when the temperature is lower than 300 C-circle. However, the quantity and average diameter of pores as well as porosity are all dramatically enhanced when the temperature ranges from 300 to 400 C-circle, and each parameter attains a maximum value when the temperature reaches 500 C-circle. This can be expected from the formation of a great amount of oil and gases through pyrolysis of solid organic matter at higher temperature. And so the spaces previously occupied by organic matter become the ones filled with pores. The newly-formed pores can be taken as the valid channels connecting the original pores. This is attested by the decrease in the quantity of pores with a diameter of 0.54-1.50 mu m and the slight increase in the amount of pores having a diameter from 1.70 to 4.10 mu m. Thus, the coalescence of mu m-sized pores enlarged the channels facilitating the fluid percolation, in favor of the injection of heated fluids and the release of oil and gases during pyrolysis.