At present, there is a growing tendency to use low cost, commercially available oil-shale ash as a building material, a chemical filling material, an adsorbent, and so forth. To obtain oil-shale ash with higher porosity, the pore structure of oil-shale ash samples obtained from different combustion modes of oil shale was measured by using a N-2 isothermal adsorption/desorption method. The surface morphology of sample particles was photographed by field emission scanning electron microscopy, and their surface fractal dimensions were computed by a simple N-2 adsorption isotherm method as well. As a result of a comparison between pore structures of oil-shale ash samples, the oil-shale ash, formed at a fast combustion mode without ash agglomeration occurring, has a larger pore volume and specific surface area because it has more pores and a rougher surface.