Twin-roll casting allows production of sheets and strips with required thickness without subsequent homogenization or hot rolling and therefore has a high potential for industrial applications. One of the drawbacks of this method is a formation of inhomogeneous structure with central segregation. Sheet prepared from 3003 aluminum alloy with 1 wt.% Mn, 0.2 wt.% Fe, 0.5 wt.% Si, 0.2 wt.% Cu and small addition of Zr (0.2 wt.%) and Cr (0.1 wt.%) was studied by X-ray computed tomography. Segregations in the material form as manganese, iron and silicon rich channels spread in the rolling direction. The material annealed at 450 °C exhibits diffusion of alloying elements from segregations into their surroundings and formation of voids or a material with lower density in the center of segregation channels due to Kirkendall effect.