We report an experimental study of the surface modification of polydimethylsiloxane (PDMS) by atmospheric pressure plasma (APP). The contact angle of a water droplet, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR) were used to analyze the modified surface and the hydrophilic stability of PDMS samples, which were mixed with different ratios of base polymer and curing agent. The modified hydrophilic surface of PDMS lasted for 20 days, the duration of our experiment. In FTIR analysis, a broad peak at 3420 cm Γ1 appeared after plasma treatment for 15 s, which corresponded to hydroxyl group formation on the PDMS surface during plasma treatment. Another new finding is that the magnitude of the peak for PDMS-05, which contained excess curing agent, was the smallest among the PDMS samples. Thus, the plasma treatment modifies the surface of the PDMS by adding hydroxyl groups and the resulting hydrophilic surface depends on the ratio of base polymer to curing agent. Moreover, SEM analysis showed that the bare PDMS-05 sample had a cracked surface, while the bare PDMS-20 was relatively smooth. This cracked or rough topology of the bare PDMS decreased with increased base polymer on reducing amount of curing agent. This improvement in the surface roughness of the plasmatreated samples may be caused by a shallow etching process that occurs during plasma treatment with oxygen gas. It constitutes an effective method for modifying the surface of PDMS without specific skills, expensive apparatus, or clean-room facilities.