Abstract: In order to improve the hydrophilicity of Poly （dimethylsiloxane） （PDMS） and steady its electroosmotic flow （EOF） properties, the air plasma generated by microwave was chosen to treat PDMS. Analysis was conducted with AFM, XPS, and contact angle before and after plasma treatment. After being treated for only 3 min, the static contact angle of PDMS almost changed to zero, which shows the hydrophilicity of PDMS is improved. As times flies, the hydrophilic surface turned to hydrophobic gradually. The XPS result indicates the modification of the PDMS to a silica-like surface （SiO_x） and AFM result shows that the air plasma treatment does not change the surface morphology of the PDMS. Compared to the reported procedures using oxygen plasma treatment under high and mediate vacuum, this approach needs equipment with simple vacuum system, and also it has a higher efficiency. The best treating conditions are as follows: the microwave power is 100 W, the total pressure in the chamber is kept at 1.0 kPa, and the flow rate of air is 20 sccm （1 sccm=1 cm³·min^-1）.