Recently, hydrodynamic chromatography(HDC) has become an important probe for determining the molecular size or molecular shape in the sub-micron range. Although a lot of studies on HDC were performed, the clear understanding on the transport behavior of polymer solutions in porous media has not been achieved yet. In this study, the flow and dynamic behavior of polymer molecules in a packed HDC column is fully analyzed by extending the molecular kinetic approach of dilute polymer solution in a confined geometry to elucidate the effects of relative particle sizes as well as the flow strength on the retention factor(R_f). R_f equation of each simple polymer model was developed, and the numerical simulation was worked out to illustrate the R_f for rigid rod(RR) polymers. Theoretical predictions were in remarkable agreement with our experimental results of xanthan gum and other published data despite of several approximations. Significant size effects were observed, and for RR model the R_f decreased with increasing the flow strength within a particular range. This feature emphasized a transition behavior from weak to strong flow due to the orientational effect of xanthan molecules. It should be noted that our major concern is restricted solely to the hydrodynamic force.