The contribution deals with the possibilities of application of the X-ray CT on rock granitic samples for an analysis of the nature and distribution of the pore space or other inhomogeneities. Common applications of X-ray CT on the compact granitic rocks lets you see only the major faults and the pores of larger volume. Summarily small pore volume and especially their fissure shapes makes visibility of them and analysis problematic. The authors applied a method of the mercury porosimetry to modify the analyzed samples and subsequently to increase highlighting the contrast fissure pore. The article shows and compares the results of X-ray CT analysis on the smaples efore and after the mercury porosimetry.