The paper deals with sodium sulphate crystallization in pore spaces of Thassos marble and presents the 3D visualization of intergranular fractures causing a deterioration of the specimen by the salt crystallization. Cylindrical marble specimen with diameter 20 mm and length 50 mm has been submitted to 15 cycles of the dipping in 14 % solution of mirabilite (Na2SO4·10 H2O) according to the STN EN 12370 standard methodical test. Using electron methods (polarization microscopy, SEM – Scanning electron microscopy, XRD – X-ray diffraction, EMPA – Electron-microprobe analysis) and by monitoring of selected physical parameters (changes in weight and changes in P-waves velocities), in first the mineral composition and microstructure of the marble have been characterized and consequently the effects induced by cyclic activity of sodium sulphate with the marble have been analysed. Mercury intrusion porosimetry (MIP) method has been used for specification of the porosity and detail pore structure. After realised 15 cycles of dipping in salt solution the presence of sodium sulphate in EDS spectrum of the marble has been documented. The sample weight has been reduced about 1.3 % and as well the speed of P-waves propagation has been reduced about 20 %. Before and after salt crystallization test the marble has been characterized using a computed microtomography (CMT), allowing to doing the image about intensity and localization of the created micro-fractures in 3D. It was identified that cyclic salt crystallization caused the decay of the marble predominantly along the dominant dolomite crystal boundaries. The hydration and dehydration reactions of the sodium sulphate and its phase transitions with cyclic changes of humidity and temperature are also discussed in the paper.