The classical Ogston-Laurent gel model of size exclusion chromatography was modified to include both dextran fibers and cross-links. The dimensions of Sephadex gel fibers were determined from the experimental data in the literature and the microstructural study of Sephadex gel. The corresponding radii of the dextran chain and the cross-linking were calculated to be 7.8 × 10^-8 and 4.8 × 10^-8 cm, respectively. The lengths of the dextran chain varies from 1.45 × 10¹²（G-200） to 9.46 × 10¹² cm/ml (G-25) while the length of the cross-link varies from 0.14 × 10¹² (G-200) to 3.18 × 10¹² cm/ml (G-25). The partial specific volume of swollen Sephadex gel material was calculated to be 0.586 ml/g.