The unidirectional porous C/C composites with different channel-diameter and channels-row spacing were prepared by a space-holder route. The effects of different channel-diameter and channels-row spacing on the mechanical properties and microstructures of the porous C/C composites were studied. Results show that increasing the channel-diameter or reducing the channels-row spacing helps to obtain finer PyC grains but significantly reduce the matrix density of the composites, and, in turn, the compressive strength of the composites. Porous C/C composites with lower porosity or higher density (e.g., S_5,2) present relatively high strength, whereas C/C porous composites with lower density or higher porosity show a pseudo-plastic fracture mode and relatively lower strength. The compression strength in the load parallel to channel direction is higher than that in the load perpendicular to channel direction of the same composite.