High-energy gamma-ray computed tomography (CT scanner system) has been used to study, for the first time, the structure of the pebble bed in terms of cross-sectional time-averaged void and solids distributions, and their radial profiles along the bed height. The pebble beds were packed with pebbles of glass spheres of diameter of d1 = 0.0127 m, d₂ = 0.0254 m and d₃ = 0.05 m in a Plexiglas cylinder with diameter of D = 0.3048 m, with D/d1 = 24, D/d₂ = 12 and D/d₃ = 6, respectively. The average void fraction of these beds were ε¯1=0.389,ε¯2=0.40 and ε¯3=0.43, respectively. It was found that the void and solids distribution depends on pebble size, the bed diameter and the packing method. The void radial profiles vary with the bed height for larger pebbles at the measured axial levels (0.0762, 0.1524, and 0.2286 m from the bed base). However, adding additional bed height on the top of the original one did not show any influence on the average voids and their radial profiles at the same measured axial levels. The experimental results indicated that void variations are normally distributed in the bulk region of the packed bed.