The elution behavior of high molar mass polystyrene (PS) in packed-column hydrodynamic chromatography (HDC) was studied at different eluent flow rates by using a multiangle laser light scattering (MALLS) detector. A chromatography mode transition from HDC to slalom chromatography (SC) was observed for high molar mass PS samples with abnormal elution behaviors. The critical polymer size R-g,R-c of the HDC to SC transition was determined by the plateau value of the R-g-V-e curve for the earliest eluting polymer fraction. The dependence of Rg, on flow rate was explained by coil-stretch transitions of the polymer molecules in elongational flow through the packed column. The transition of chromatography mode may provide a new method to study the coil-stretch transition of polymers in elongational flow through packed beds. For high molar mass polymer samples, HDC must be performed at flow rates sufficiently low that the elongational rate in the column is less than the critical strain rate for coil-stretch transitions to occur for all species in the sample.