Internal erosion is a complex phenomenon, which is a main problem for the long-term stability of earth hydraulic structure impacted by seepage. One particular phenomenon of internal erosion, the migration of the finest soil particles through the surrounding soil matrix formed by the granular skeleton, is called suffusion. The occurrence of suffusion usually causes modifications of the microstructure and changes in the physical, hydraulic and mechanical properties of the soil. The mechanism of suffusion development and its consequences on the mechanical behaviour are usually addressed at a macroscopic scale in experimental analysis while they most probably arise from microscopic processes. In this paper, the investigation of suffusion development and mechanical properties of eroded soil are linked to the microscopic scale. For this purpose, suffusion tests are firstly performed on granular soil sample using a newly developed erosion permeameter, and then subjected to a mechanical loading; while high resolution x-ray computed tomography is used to capture the micro-structural evolution during the erosion process. The results obtained from x-ray CT analyse show that the erosion process is highly heterogeneous in term of fine content, void ratio and inter-granular void ratio, with the presence of random preferential flow paths. The presence of shear deformation and non-uniform volumetric strain in the sample after suffusion process are also remarkable. These features may cause counter-intuitive results on the mechanical behaviour deduced from an average stress-strain in triaxial test for eroded soil.