A hydrophilic poly(dimethylsiloxane) (PDMS) microchip with stable electroosmotic flow (EOF) was prepared by a simple and reproducible coating procedure with silica nanoparticles. The microchannel wall of PDMS chip was coated with a layer of poly(diallyldimethylammonium chloride) (PDDA) and then collected silica nanoparticles. The assembly was followed by contact angle, charge-coupled device (CCD) imaging, electroosmotic flow (EOF) measurements and electrophoretic separation experiments. Contact angle measurements revealed the coated surface was hydrophilic; the water contact angle for coated chips was 64 degrees compared with a water contact angle for native PDMS chips of 113 degrees . CCD images indicated a substantially more hydrophilic microchannel than native PDMS. We carried out a comparison and concluded that the EOF values on the coated PDMS chip were close to those values on the glass chip above pH 7.0. The coated channel had an excellent stability and reproducibility, RSD of EOF values (n=6) on native and coated PDMS microchip was 1.58 and 0.57%, respectively. Separation of dopamine and epinephrine was performed on the coated chip generated 1.40 x 10(5), 1.39 x 10(5) theoretical plates/m compared with the native PDMS chip of 0.79 x 10(5), 0.88 x 10(5), high resolution of 1.7 was achieved with a channel of 3.60 cm length.