Dextran, a main constituent of Sephadex gel, is a linear homopolymer (polysaccharide) composed almost exclusively of the monomeric unit, ${alpha}$-D-glucopyranose, linked mainly by 1-6 glucosidic bonds. As a basic building block of dextran, the conformation of ${alpha}$-gentiobiose (6-O-${alpha}$-D-glucopyranosyl-${alpha}$-D-glucopyranose) in vacuum was studied by the technique of the consistent force field. We presented the procedures of identifying the position of each atoms in a reference coordinate system. The molecular structure of a Sephadex gel was simulated as two sets of cylinders: one for a unit of ${alpha}$-gentiobiose and the other for a cross-linking bridge. The structural dimension of a dextran gel (Sephadex G-25) was expressed by the radius and the length of those cylinders: a cylinder with a radius at 2.54 $dot{A}$ and a length of 5.61 $dot{A}$ for ${alpha$}-gentiobiose and a cylinder with a radius of 1.57 $dot{A}$ a length of 4.84 $dot{A}$ for the cross-linking bridge.