We demonstrated DNA preconcentration and separation in the presence of electroosmotic flow (EOF) using poly(ethylene oxide) (PEO) solutions. After injecting large volumes of DNA samples into a capillary filled with free tris(hydroxymethyl)amino-methane (Tris)-borate (TB) buffers, PEO solutions entered the capillary by EOF and acted as sieving matrices. In contrast to conventional methods (in the absence of EOF), controlling the EOF was also useful for resolution optimization. We have found that PEO adsorption on the capillary wall was more pronounced when low ionic strength buffers were used. Thus, the EOF decreased with increasing injection length, which led to longer migration times and changes in resolution and stacking efficiency. All resolution values were higher than 1.5 when 1.0 μg/mL DNA samples were injected at 240 V/cm for 60 s (0.67 μL). In addition, as low as 0.015 μg/mL DNA samples (an about 66-fold increase in sensitivity) were detected when the injection was performed at 250 V/cm for 60 s.